U.S. patent number 10,396,361 [Application Number 16/069,963] was granted by the patent office on 2019-08-27 for nonaqueous lithium-type power storage element.
This patent grant is currently assigned to Asahi Kasei Kabushiki Kaisha. The grantee listed for this patent is Asahi Kasei Kabushiki Kaisha. Invention is credited to Takeshi Kamijo, Keita Kusuzaka, Kimiya Murakami, Nobuhiro Okada, Kazuteru Umetsu.
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United States Patent |
10,396,361 |
Kamijo , et al. |
August 27, 2019 |
Nonaqueous lithium-type power storage element
Abstract
The disclosure provides a nonaqueous lithium power storage
element containing a positive electrode, a negative electrode, a
separator and a lithium ion-containing nonaqueous electrolytic
solution.
Inventors: |
Kamijo; Takeshi (Tokyo,
JP), Kusuzaka; Keita (Tokyo, JP), Umetsu;
Kazuteru (Tokyo, JP), Murakami; Kimiya (Tokyo,
JP), Okada; Nobuhiro (Tokyo, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Asahi Kasei Kabushiki Kaisha |
Tokyo |
N/A |
JP |
|
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Assignee: |
Asahi Kasei Kabushiki Kaisha
(Tokyo, JP)
|
Family
ID: |
59362454 |
Appl.
No.: |
16/069,963 |
Filed: |
January 20, 2017 |
PCT
Filed: |
January 20, 2017 |
PCT No.: |
PCT/JP2017/002031 |
371(c)(1),(2),(4) Date: |
July 13, 2018 |
PCT
Pub. No.: |
WO2017/126697 |
PCT
Pub. Date: |
July 27, 2017 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20190027754 A1 |
Jan 24, 2019 |
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Foreign Application Priority Data
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|
|
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Jan 22, 2016 [JP] |
|
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2016-010895 |
Aug 8, 2016 [JP] |
|
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2016-155562 |
Aug 8, 2016 [JP] |
|
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2016-155917 |
Sep 30, 2016 [JP] |
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2016-192605 |
Sep 30, 2016 [JP] |
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2016-192692 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M
10/0585 (20130101); H01M 4/587 (20130101); H01M
10/0525 (20130101); H01M 4/62 (20130101); H01G
11/50 (20130101); H01M 10/052 (20130101); H01G
11/24 (20130101); H01G 11/32 (20130101); H01G
11/70 (20130101); H01M 4/1393 (20130101); H01G
11/38 (20130101); H01G 11/06 (20130101); H01M
10/0567 (20130101); H01M 10/4235 (20130101); Y02T
10/70 (20130101); Y02E 60/13 (20130101); Y02T
10/7022 (20130101); H02J 9/00 (20130101); H01G
11/46 (20130101) |
Current International
Class: |
H01G
11/32 (20130101); H01G 11/70 (20130101); H01M
10/0525 (20100101); H01M 10/42 (20060101); H01M
4/587 (20100101); H01G 11/38 (20130101); H01G
11/50 (20130101); H01M 4/133 (20100101); H01M
4/62 (20060101); H01G 11/06 (20130101); H01M
4/1393 (20100101); H01M 10/052 (20100101); H01M
10/0567 (20100101); H01M 10/0585 (20100101); H01G
11/24 (20130101); H02J 9/00 (20060101); H01G
11/46 (20130101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2219247 |
|
Aug 2010 |
|
EP |
|
H04-328278 |
|
Nov 1992 |
|
JP |
|
2001-167767 |
|
Jun 2001 |
|
JP |
|
2004-014300 |
|
Jan 2004 |
|
JP |
|
2004-095188 |
|
Mar 2004 |
|
JP |
|
2004-362859 |
|
Dec 2004 |
|
JP |
|
2008-177263 |
|
Jul 2008 |
|
JP |
|
2008-181830 |
|
Aug 2008 |
|
JP |
|
2012-174437 |
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Sep 2012 |
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JP |
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2012-212629 |
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Nov 2012 |
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JP |
|
2014-199723 |
|
Oct 2014 |
|
JP |
|
2016-012620 |
|
Jan 2016 |
|
JP |
|
2011/058748 |
|
May 2011 |
|
WO |
|
2014/088074 |
|
Jun 2014 |
|
WO |
|
2016/006632 |
|
Jan 2016 |
|
WO |
|
Other References
Barrett et al., "The Determination of Pore Volume and Area
Distributions in Porous Substances: Computations from Nitrogen
Isotherms," The Journal of the American Chemical Society, 73:
373-380 (1951). cited by applicant .
Lippens et al., "Studies on Pore Systems in Catalysts: The t
Method," Journal of Catalysts, 4: 319-323 (1965). cited by
applicant .
Mikhail et al., "Investigations of a Complete Pore Structure
Analysis: Analysis of Micropores," Journal of Colloid and Interface
Science, 26: 45-53 (1968). cited by applicant .
International Search Report issued in corresponding International
Patent Application No. PCT/JP2017/002031 dated Apr. 18, 2017. cited
by applicant .
International Preliminary Report on Patentability and Written
Opinion issued in corresponding International Patent Application
No. PCT/JP2017/002031 dated Aug. 2, 2018. cited by applicant .
Decision to Grant issued in corresponding Japanese Patent
Application No. 2017-509066 dated Jan. 16, 2018. cited by applicant
.
Supplemental European Search Report issued related European Patent
Application No. 17741574.2 dated Jan. 4, 2019. cited by
applicant.
|
Primary Examiner: Crepeau; Jonathan
Attorney, Agent or Firm: Morgan, Lewis & Bockius LLP
Claims
The invention claimed is:
1. A nonaqueous lithium power storage element comprising a positive
electrode, a negative electrode, a separator and a lithium
ion-containing nonaqueous electrolytic solution, wherein the
negative electrode has a negative electrode power collector, and a
negative electrode active material layer containing a negative
electrode active material, provided on one or both sides of the
negative electrode power collector, the positive electrode has a
positive electrode power collector, and a positive electrode active
material layer containing a positive electrode active material,
provided on one or both sides of the positive electrode power
collector, in the solid .sup.7Li-NMR spectrum of the positive
electrode active material layer, the relationship
1.04.ltoreq.b/a.ltoreq.5.56 is satisfied, where "a" is the peak
area from -40 ppm to 40 ppm obtained by measuring with a repeated
latency of 10 seconds, and "b" is the peak area from -40 ppm to 40
ppm obtained by measuring with a repeated latency of 3,000 seconds,
the positive electrode active material layer contains a lithium
compound other than the positive electrode active material, the
positive electrode active material is an activated carbon, and the
lithium compound is at least one selected from the group consisting
of lithium carbonate, lithium oxide and lithium hydroxide.
2. The nonaqueous lithium power storage element according to claim
1, wherein the amount of lithium in the positive electrode, as
calculated by the peak area from -40 ppm to 40 ppm in the solid
.sup.7Li-NMR spectrum of the positive electrode active material
layer, is 1 mmol/g to 30 mmol/g per unit weight of the positive
electrode active material layer.
3. The nonaqueous lithium power storage element according to claim
2, wherein the positive electrode contains one or more compounds
selected from the group consisting of compounds represented by the
following formulas (1) to (3): LiX.sup.1--OR.sup.1O--X.sup.2Li (1)
wherein, in formula (1), R.sup.1 is an alkylene group of 1 to 4
carbon atoms or a halogenated alkylene group of 1 to 4 carbon
atoms, and X.sup.1 and X.sup.2 respectively and independently
represent --(COO).sub.n (where n is 0 or 1),
LiX.sup.1--OR.sup.1O--X.sup.2R.sup.2 (2) wherein, in formula (2),
R.sup.1 is an alkylene group of 1 to 4 carbon atoms or a
halogenated alkylene group of 1 to 4 carbon atoms, R.sup.2 is
hydrogen, an alkyl group of 1 to 10 carbon atoms, a mono- or
polyhydroxyalkyl group of 1 to 10 carbon atoms, an alkenyl group of
2 to 10 carbon atoms, a mono- or polyhydroxyalkenyl group of 2 to
10 carbon atoms, a cycloalkyl group of 3 to 6 carbon atoms, or an
aryl group, and X.sup.1 and X.sup.2 respectively and independently
represent --(COO).sub.n (where n is 0 or 1), and
R.sup.2X.sup.1--OR.sup.1O--X.sup.2R.sup.3 (3) wherein, in formula
(3), R.sup.1 is an alkylene group of 1 to 4 carbon atoms or a
halogenated alkylene group of 1 to 4 carbon atoms, R.sup.2 and
R.sup.3 respectively and independently represent hydrogen, an alkyl
group of 1 to 10 carbon atoms, a polyhydroxyalkyl group of 1 to 10
carbon atoms, an alkenyl group of 2 to 10 carbon atoms, a mono- or
polyhydroxyalkenyl group of 2 to 10 carbon atoms, a cycloalkyl
group of 3 to 6 carbon atoms or an aryl group, and X.sup.1 and
X.sup.2 respectively and independently represent --(COO).sub.n
(where n is 0 or 1), in an amount of 1.60.times.10.sup.-4 mol/g to
300.times.10.sup.-4 mol/g per unit weight of the positive electrode
active material layer.
4. The nonaqueous lithium power storage element according to claim
1, wherein the mean particle diameter X.sub.1 of the lithium
compound is 0.1 .mu.m to 10 .mu.m.
5. The nonaqueous lithium power storage element according to claim
4, wherein 2 .mu.m.ltoreq.Y.sub.1.ltoreq.20 .mu.m and
X.sub.1<Y.sub.1 are satisfied, where Y.sub.1 is the mean
particle diameter of the positive electrode active material, and
the content ratio of the lithium compound in the positive electrode
is 1 weight % to 50 weight % based on the total weight of the
positive electrode active material layer.
6. The nonaqueous lithium power storage element according to claim
1, wherein the content ratio of the lithium compound in the
positive electrode is 1weight % to 20 weight % based on the total
weight of the positive electrode active material layer.
7. The nonaqueous lithium power storage element according to claim
1, wherein the mean distance between the centers of gravity of
voids of the negative electrode active material layer, as obtained
by SEM of a cross-section of the negative electrode active material
layer, is 1 .mu.m to 10 .mu.m.
8. The nonaqueous lithium power storage element according to claim
7, wherein r.sub.p/r.sub.a is 0.10 to 1.10, where r.sub.p is the
mean distance between the centers of gravity of the voids and
r.sub.a is the mean particle diameter of the negative electrode
active material.
9. The nonaqueous lithium power storage element according to claim
1, wherein the negative electrode active material contains a
graphite-based carbon material, the negative electrode active
material layer intercalates lithium ion, and in the solid
.sup.7Li-NMR spectrum of the negative electrode active material
layer, in the spectral range of -10 ppm to 35 ppm, the maximum
value of the peaks is between 4 ppm to 30 ppm, and the amount of
lithium as calculated by the peak area from 4 ppm to 30 ppm is 0.10
mmol/g to 10.0 mmol/g per unit weight of the negative electrode
active material layer.
10. The nonaqueous lithium power storage element according to claim
1, wherein the BET specific surface area per unit volume of the
negative electrode active material layer is 1 m.sup.2/cc to 50
m.sup.2/cc.
11. The nonaqueous lithium power storage element according to claim
1, wherein the mean pore size of the negative electrode active
material layer is 2 nm to 20 nm.
12. The nonaqueous lithium power storage element according to claim
1, wherein the mean particle diameter of the negative electrode
active material is 1 .mu.m to 10 .mu.m.
13. The nonaqueous lithium power storage element according to claim
1, wherein the negative electrode active material contains a
composite carbon material comprising a graphite material and a
carbonaceous material.
14. The nonaqueous lithium power storage element according to claim
1, wherein the doping amount of lithium ion in the negative
electrode active material is 50 mAh/g to 700 mAh/g per unit weight
of the negative electrode active material.
15. The nonaqueous lithium power storage element according to claim
1, wherein the BET specific surface area of the negative electrode
active material is 1 m.sup.2/g to 50 m.sup.2/g.
16. The nonaqueous lithium power storage element according to claim
1, wherein the doping amount of lithium ion in the negative
electrode active material is 530 mAh/g to 2,500 mAh/g per unit
weight of the negative electrode active material.
17. The nonaqueous lithium power storage element according to claim
1, wherein the BET specific surface area of the negative electrode
active material is 100 m.sup.2/g to 1,500 m.sup.2/g.
18. The nonaqueous lithium power storage element according to claim
1, wherein the activated carbon satisfies 0.3
<V.sub.1.ltoreq.0.8 and 0.5.ltoreq.V.sub.2 .ltoreq.1.0, where
V.sub.1 (cc/g) is the mesopore volume due to pores with diameters
of 20 .ANG. to 500 .ANG. as calculated by the BJH method, and
V.sub.2 (cc/g) is the micropore volume due to pores with diameters
of smaller than 20 .ANG. as calculated by the MP method, and has a
specific surface area of 1,500 m.sup.2/g to 3,000 m.sup.2/g, as
measured by the BET method.
19. The nonaqueous lithium power storage element according to claim
1, wherein the activated carbon satisfies 0.8
<V.sub.1.ltoreq.2.5 and 0.8<V.sub.2 3.0,where V.sub.1 (cc/g)
is the mesopore volume due to pores with diameters of 20 .ANG. to
500 .ANG. as calculated by the BJH method, and V.sub.2 (cc/g) is
the micropore volume due to pores with diameters of smaller than 20
.ANG. as calculated by the MP method, and has a specific surface
area of 2,300 m.sup.2/g to 4,000 m.sup.2/g, as measured by the BET
method.
20. The nonaqueous lithium power storage element according to claim
1, wherein the positive electrode power collector and the negative
electrode power collector are metal foils without
through-holes.
21. The nonaqueous lithium power storage element according to claim
1, wherein the following (a) and (b) are satisfied for the
nonaqueous lithium power storage element: (a) the product of Ra and
F, RaF, is 0.3 to 3.0, (b) E/V is 15 to 50, where Ra (.OMEGA.) is
the initial internal resistance, F (F) is the electrostatic
capacitance, E (Wh) is the electrical energy and V (L) is the
volume of the power storage element.
22. The nonaqueous lithium power storage element according to claim
1, wherein, for charge/discharge cycling of the nonaqueous lithium
power storage element conducted 60,000 times at an environmental
temperature of 25.degree. C. and a rate of 300C, in a cell voltage
range from 2.2 V to 3.8 V, Rb/Ra is 0.9 to 2.0, where Rb (.OMEGA.)
is the internal resistance after the charge/discharge cycling and
Ra (.OMEGA.) is the internal resistance before the charge/discharge
cycling.
23. A power storage module containing the nonaqueous lithium power
storage element according to claim 1.
24. A power regenerating system containing the nonaqueous lithium
power storage element according to claim 1.
25. A power load-leveling system containing the nonaqueous lithium
power storage element according to claim 1.
26. An uninterruptable power source system containing the
nonaqueous lithium power storage element according to claim 1.
27. A non-contact power supply system containing the nonaqueous
lithium power storage element according to claim 1.
28. An energy harvesting system containing the nonaqueous lithium
power storage element according to claim 1.
29. A power storage system containing the nonaqueous lithium power
storage element according to claim 1.
30. The nonaqueous lithium power storage element according to claim
1, wherein the positive electrode active material layer further
contains a conductive filler, binder or dispersion stabilizer.
31. The nonaqueous lithium power storage element according to claim
1, wherein the positive electrode power collector is an aluminum
foil.
Description
FIELD
The present invention relates to a nonaqueous lithium power storage
element.
BACKGROUND
In recent years, with an aim toward effective utilization of energy
for greater environmental conservation and reduced usage of
resources, a great deal of attention is being directed to power
smoothing systems for wind power generation or overnight charging
electric power storage systems, household dispersive power storage
systems based on solar power generation technology, and power
storage systems for electric vehicles and the like.
The number one requirement for cells used in such power storage
systems is high energy density. The development of lithium ion
batteries is therefore advancing at a rapid pace, as an effective
strategy for cells with high energy density that can meet this
requirement.
The second requirement is a high output characteristic. A high
power discharge characteristic is required for a power storage
system during acceleration in, for example, a combination of a high
efficiency engine and a power storage system (such as in a hybrid
electric vehicle), or a combination of a fuel cell and a power
storage system (such as in a fuel cell electric vehicle).
Electrical double layer capacitors and nickel-metal hydride
batteries are currently under development as high output power
storage devices.
Electrical double layer capacitors that employ activated carbon in
the electrodes have output characteristics of about 0.5 to 1 kW/L.
Such electrical double layer capacitors have high durability (cycle
characteristics and high-temperature storage characteristics), and
have been considered optimal devices in fields where the high
output mentioned above is required. However, their energy densities
are no greater than about 1 to 5 Wh/L. A need therefore exists for
even higher energy density.
On the other hand, nickel-metal hydride batteries employed in
existing hybrid electric vehicles exhibit high output equivalent to
electrical double layer capacitors, and have energy densities of
about 160 Wh/L. Still, research is being actively pursued toward
further increasing their energy density and output, and increasing
their durability (especially stability at high temperatures).
Research is also advancing toward increased outputs for lithium ion
batteries as well. For example, lithium ion batteries are being
developed that yield high output exceeding 3 kW/L at 50% depth of
discharge (a value representing the state of the percentage of
discharge of the service capacity of a power storage element).
However, the energy density is 100 Wh/L or lower, and the design is
such that the high energy density, which is the major feature of a
lithium ion battery, is reduced. Also, the durability (especially
cycle characteristic and high-temperature storage characteristic)
is inferior to that of an electrical double layer capacitor. In
order to provide practical durability, therefore, these are used
with a depth of discharge in a narrower range than 0 to 100%.
Because the usable capacity is even lower, research is actively
being pursued toward further increasing durability.
There is a strong demand for implementation of power storage
elements exhibiting high energy density, high output
characteristics and durability, as mentioned above. Nevertheless,
the existing power storage elements mentioned above have their
advantages and disadvantages. New power storage elements are
therefore desired that can meet these technical requirements.
Promising candidates are power storage elements known as lithium
ion capacitors, which are being actively developed in recent
years.
The energy of a capacitor is represented as 1/2CV.sup.2 (where C is
electrostatic capacitance and V is voltage).
A lithium ion capacitor is a type of power storage element using a
nonaqueous electrolytic solution comprising a lithium salt (or,
"nonaqueous lithium power storage element"), wherein
charge/discharge is accomplished by: non-Faraday reaction by
adsorption/desorption of anions similar to an electrical double
layer capacitor at about 3 V or higher, at the positive electrode;
and Faraday reaction by intercalation/release of lithium ions
similar to a lithium ion battery, at the negative electrode.
To summarize these electrode materials and their characteristics:
when charge/discharge is carried out using a material such as
activated carbon as an electrode, by adsorption and desorption of
ions on the activated carbon surface (non-Faraday reaction), it is
possible to obtain high output and high durability, but with lower
energy density (for example, one-fold). On the other hand, when
charge/discharge is carried out by Faraday reaction using an oxide
or carbon material as the electrode, the energy density is higher
(for example, 10 times that of non-Faraday reaction using activated
carbon), but then durability and output characteristic become
problems.
Electrical double layer capacitors that combine these electrode
materials employ activated carbon as the positive electrode and
negative electrode (energy density: 1.times.), and carry out
charge/discharge by non-Faraday reaction at both the positive and
negative electrodes, and are characterized by having high output
and high durability, but also low energy density (positive
electrode: one-fold.times.negative electrode: one-fold=1).
Lithium ion secondary batteries use a lithium transition metal
oxide (energy density: 10-fold) for the positive electrode and a
carbon material (energy density: 10-fold) for the negative
electrode, carrying out charge/discharge by Faraday reaction at
both the positive and negative electrodes, but while their energy
density is high (positive electrode: 10-fold.times.negative
electrode: 10-fold=100), they have problems in terms of output
characteristic and durability. In addition, the depth of discharge
must be restricted in order to satisfy the high durability required
for hybrid electric vehicles, and with lithium ion secondary
batteries only 10 to 50% of the energy can be utilized.
A lithium ion capacitor is a new type of asymmetric capacitor that
employs activated carbon (energy density: 1.times.) for the
positive electrode and a carbon material (energy density: 10-fold)
for the negative electrode, and it is characterized by carrying out
charge/discharge by non-Faraday reaction at the positive electrode
and Faraday reaction at the negative electrode, and thus having the
characteristics of both an electrical double layer capacitor and a
lithium ion secondary battery. It therefore exhibits high output
and high durability, while also having high energy density
(positive electrode: 1x negative electrode: 10-fold=10) and
requiring no restrictions on depth of discharge as with a lithium
ion secondary battery.
The purposes for which lithium ion capacitors are used include
power storage elements for railways, construction machines and
automobiles, for example. These uses require both a high
input/output characteristic and a high-load charge/discharge cycle
characteristic.
In PTL 1 there is proposed a lithium ion secondary battery using a
positive electrode containing lithium carbonate in the positive
electrode, and having a current shielding mechanism that operates
in response to increased internal pressure in the battery.
In PTL 2 there is proposed a lithium ion secondary battery
employing a lithium complex oxide such as lithium manganate as the
positive electrode, and with reduced elution of manganese by
including lithium carbonate in the positive electrode.
In PTL 3 there is proposed a method of causing restoration of the
capacitance of a deteriorated power storage element by oxidizing
different lithium compounds as coated oxides at the positive
electrode.
CITATION LIST
Patent Literature
[PTL 1] Japanese Unexamined Patent Publication HEI No. 4-328278
[PTL 2] Japanese Unexamined Patent Publication No. 2001-167767 [PTL
3] Japanese Unexamined Patent Publication No. 2012-174437
SUMMARY
Technical Problem
The present inventors have found that the high-load
charge/discharge cycle characteristic can be increased by adding a
lithium compound to the positive electrode, as exemplified by PTLs
1 to 3. However, as the lithium compound content of the positive
electrode increases, the resistance of the nonaqueous lithium power
storage element using it also increases, and this has made it
difficult to obtain a satisfactory input/output characteristic.
In light of this situation, the problem to be solved by the present
invention is that of providing a nonaqueous lithium power storage
element having a high input/output characteristic and a high-load
charge/discharge cycle characteristic.
Solution to Problem
As a result of much ardent research and experimentation focused on
solving this problem, the present inventors have found that by
specifying the range for the ratio b/a, where in the solid
.sup.7Li-NMR spectrum of the positive electrode active material
layer, "a" is the peak area from -40 ppm to 40 ppm obtained by
measuring with a repeated latency of 10 seconds, and "b" is the
peak area from -40 ppm to 40 ppm obtained by measuring with a
repeated latency of 3,000 seconds, it is possible to exhibit a high
input/output characteristic and a high-load charge/discharge cycle
characteristic, and the present invention has thereupon been
completed.
Specifically, the present invention provides the following.
[1]
A nonaqueous lithium power storage element comprising a positive
electrode, a negative electrode, a separator and a lithium
ion-containing nonaqueous electrolytic solution, wherein
the negative electrode has a negative electrode power collector,
and a negative electrode active material layer containing a
negative electrode active material, provided on one or both sides
of the negative electrode power collector,
the positive electrode has a positive electrode power collector,
and a positive electrode active material layer containing a
positive electrode active material, provided on one or both sides
of the positive electrode power collector, and
in the solid .sup.7Li-NMR spectrum of the positive electrode active
material layer, the relationship 1.04.ltoreq.b/a.ltoreq.5.56 is
satisfied, where "a" is the peak area from -40 ppm to 40 ppm
obtained by measuring with a repeated latency of 10 seconds, and
"b" is the peak area from -40 ppm to 40 ppm obtained by measuring
with a repeated latency of 3,000 seconds.
[2]
The nonaqueous lithium power storage element according to [1],
wherein the amount of lithium in the positive electrode, as
calculated by the peak area from -40 ppm to 40 ppm in the solid
.sup.7Li-NMR spectrum of the positive electrode active material
layer, is 1 mmol/g to 30 mmol/g per unit weight of the positive
electrode active material layer.
[3]
The nonaqueous lithium power storage element according to [2],
wherein
the positive electrode contains one or more compounds selected from
the group consisting of compounds represented by the following
formulas (1) to (3):
[Chem. 1] LiX.sup.1--OR.sup.1O--X.sup.2Li (1) {in formula (1),
R.sup.1 is an alkylene group of 1 to 4 carbon atoms or a
halogenated alkylene group of 1 to 4 carbon atoms, and X.sup.1 and
X.sup.2 respectively and independently represent --(COO).sub.n
(where n is 0 or 1)}, [Chem. 2]
LiX.sup.1--OR.sup.1O--X.sup.2R.sup.2 (2) {in formula (2), R.sup.1
is an alkylene group of 1 to 4 carbon atoms or a halogenated
alkylene group of 1 to 4 carbon atoms, R.sup.2 is hydrogen, an
alkyl group of 1 to 10 carbon atoms, a mono- or polyhydroxyalkyl
group of 1 to 10 carbon atoms, an alkenyl group of 2 to 10 carbon
atoms, a mono- or polyhydroxyalkenyl group of 2 to 10 carbon atoms,
a cycloalkyl group of 3 to 6 carbon atoms, or an aryl group, and
X.sup.1 and X.sup.2 respectively and independently represent
--(COO).sub.n (where n is 0 or 1)}, and [Chem. 3]
R.sup.2X.sup.1--OR.sup.1O--X.sup.2R.sup.3 (3) {in formula (3),
R.sup.1 is an alkylene group of 1 to 4 carbon atoms or a
halogenated alkylene group of 1 to 4 carbon atoms, R.sup.2 and
R.sup.3 respectively and independently represent hydrogen, an alkyl
group of 1 to 10 carbon atoms, a polyhydroxyalkyl group of 1 to 10
carbon atoms, an alkenyl group of 2 to 10 carbon atoms, a mono- or
polyhydroxyalkenyl group of 2 to 10 carbon atoms, a cycloalkyl
group of 3 to 6 carbon atoms or an aryl group, and X.sup.1 and
X.sup.2 respectively and independently represent --(COO).sub.n
(where n is 0 or 1)}, in an amount of 1.60.times.10.sup.-4 mol/g to
300.times.10.sup.-4 mol/g per unit weight of the positive electrode
material layer. [4]
The nonaqueous lithium power storage element according to any one
of [1] to [3], wherein the positive electrode comprises a lithium
compound other than the positive electrode active material.
[5]
The nonaqueous lithium power storage element according to [4],
wherein the mean particle diameter X.sub.1 of the lithium compound
is 0.1 .mu.m to 10 .mu.m.
[6]
The nonaqueous lithium power storage element according to [5],
wherein 2 .mu.m.ltoreq.Y.sub.1.ltoreq.20 .mu.m and
X.sub.1<Y.sub.1 are satisfied, where Y.sub.1 is the mean
particle diameter of the positive electrode active material, and
the content ratio of the lithium compound in the positive electrode
is 1 weight % to 50 weight % based on the total weight of the
positive electrode active material layer.
[7]
The nonaqueous lithium power storage element according to any one
of [4] to [6], wherein the content ratio of the lithium compound in
the positive electrode is 1 weight % to 20 weight % based on the
total weight of the positive electrode active material layer.
[8]
The nonaqueous lithium power storage element according to any one
of [4] to [7], wherein the lithium compound is one or more types
selected from the group consisting of lithium carbonate, lithium
oxide and lithium hydroxide.
[9]
The nonaqueous lithium power storage element according to any one
of [4] to [8], wherein the mean distance between the centers of
gravity of the voids, as obtained by SEM of a cross-section of the
negative electrode active material layer, is 1 .mu.m to 10
.mu.m.
[10]
The nonaqueous lithium power storage element according to [9],
wherein r.sub.p/r.sub.a is 0.10 to 1.10, where r.sub.p is the mean
distance between the centers of gravity of the voids and r.sub.a is
the mean particle diameter of the negative electrode active
material.
[11]
The nonaqueous lithium power storage element according to any one
of [1] to [10], wherein
the negative electrode active material contains a graphite-based
carbon material,
the negative electrode active material layer intercalates lithium
ion, and
in the solid .sup.7Li-NMR spectrum of the negative electrode active
material layer, in the spectral range of -10 ppm to 35 ppm, the
maximum value of the peaks is between 4 ppm to 30 ppm, and the
amount of lithium as calculated by the peak area from 4 ppm to 30
ppm is 0.10 mmol/g to 10.0 mmol/g per unit weight of the negative
electrode active material layer.
[12]
The nonaqueous lithium power storage element according to any one
of [1] to [11], wherein the BET specific surface area per unit
volume of the negative electrode active material layer is 1
m.sup.2/cc to 50 m.sup.2/cc.
[13]
The nonaqueous lithium power storage element according to any one
of [1] to [12], wherein the mean pore size of the negative
electrode active material layer is 2 nm to 20 nm.
[14]
The nonaqueous lithium power storage element according to any one
of [1] to [13], wherein the mean particle diameter of the negative
electrode active material is 1 .mu.m to 10 .mu.m.
[15]
The nonaqueous lithium power storage element according to any one
of [1] to [14], wherein the negative electrode active material
contains a composite carbon material comprising a graphite material
and a carbonaceous material.
[16]
The nonaqueous lithium power storage element according to any one
of [1] to [15], wherein the doping amount of lithium ion in the
negative electrode active material is 50 mAh/g to 700 mAh/g per
unit weight of the negative electrode active material.
[17]
The nonaqueous lithium power storage element according to any one
of [1] to [16], wherein the BET specific surface area of the
negative electrode active material is 1 m.sup.2/g to 50
m.sup.2/g.
[18]
The nonaqueous lithium power storage element according to any one
of [1] to [8], wherein the doping amount of lithium ion in the
negative electrode active material is 530 mAh/g to 2,500 mAh/g per
unit weight of the negative electrode active material.
[19]
The nonaqueous lithium power storage element according to any one
of [1] to [8] and [18], wherein the BET specific surface area of
the negative electrode active material is 100 m.sup.2/g to 1,500
m.sup.2/g.
[20]
The nonaqueous lithium power storage element according to any one
of [1] to [19], wherein the positive electrode active material in
the positive electrode active material layer is activated carbon
satisfying 0.3<V.sub.1.ltoreq.0.8 and
0.5.ltoreq.V.sub.2.ltoreq.1.0, where V.sub.1 (cc/g) is the mesopore
volume due to pores with diameters of 20 .ANG. to 500 .ANG. as
calculated by the BJH method, and V.sub.2 (cc/g) is the micropore
volume due to pores with diameters of smaller than 20 .ANG. as
calculated by the MP method, and has a specific surface area of
1,500 m.sup.2/g to 3,000 m.sup.2/g, as measured by the BET
method.
[21]
The nonaqueous lithium power storage element according to any one
of [1] to [19], wherein the positive electrode active material in
the positive electrode active material layer is activated carbon
satisfying 0.8<V.sub.1.ltoreq.2.5 and 0.8<V.ltoreq.3.0, where
V.sub.1 (cc/g) is the mesopore volume due to pores with diameters
of 20 .ANG. to 500 .ANG. as calculated by the BJH method, and
V.sub.2 (cc/g) is the micropore volume due to pores with diameters
of smaller than 20 .ANG. as calculated by the MP method, and has a
specific surface area of 2,300 m.sup.2/g to 4,000 m.sup.2/g, as
measured by the BET method.
[22]
The nonaqueous lithium power storage element according to any one
of [1] to [21], wherein the positive electrode power collector and
the negative electrode power collector are metal foils without
through-holes.
[23]
The nonaqueous lithium power storage element according to any one
of [1] to [22], wherein the following (a) and (b) are satisfied for
the nonaqueous lithium power storage element:
(a) the product of Ra and F, RaF, is 0.3 to 3.0,
(b) EN is 15 to 50,
where Ra (.OMEGA.) is the initial internal resistance, F (F) is the
electrostatic capacitance, E (Wh) is the electrical energy and V
(L) is the volume of the power storage element.
[24]
The nonaqueous lithium power storage element according to any one
of [1] to [23], wherein, for charge/discharge cycling of the
nonaqueous lithium power storage element conducted 60,000 times at
an environmental temperature of 25.degree. C. and a rate of 300 C,
in a cell voltage range from 2.2 V to 3.8 V, Rb/Ra is 0.9 to 2.0,
where Rb (.OMEGA.) is the internal resistance after the
charge/discharge cycling and Ra (.OMEGA.) is the internal
resistance before the charge/discharge cycling.
[25]
A power storage module containing a nonaqueous lithium power
storage element according to any one of [1] to [24].
[26]
A power regenerating system containing a nonaqueous lithium power
storage element according to any one of [1] to [24].
[27]
A power load-leveling system containing a nonaqueous lithium power
storage element according to any one of [1] to [24].
[28]
An uninterruptable power source system containing a nonaqueous
lithium power storage element according to any one of [1] to
[24].
[29]
A non-contact power supply system containing a nonaqueous lithium
power storage element according to any one of [1] to [24].
[30]
An energy harvesting system containing a nonaqueous lithium power
storage element according to any one of [1] to [24].
[31]
A power storage system containing a nonaqueous lithium power
storage element according to any one of [1] to [24].
Advantageous Effects of Invention
The nonaqueous lithium power storage element of the invention
exhibits a high input/output characteristic and a high-load
charge/discharge cycle characteristic.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is an illustration of the geometric area S.sub.ano of the
flat section and the total film thickness t.sub.ano of a measuring
sample, for determining the volume
V.sub.ano=S.sub.ano.times.t.sub.ano of a negative electrode active
material layer.
DESCRIPTION OF EMBODIMENTS
An embodiment of the invention (hereunder referred to as "this
embodiment") will now be explained in detail as an example, with
the understanding that the invention is not limited to this
embodiment. The upper limits and lower limits for the numerical
ranges in this embodiment may be combined as desired to constitute
any desired numerical ranges.
A nonaqueous lithium power storage element generally comprises a
positive electrode, a negative electrode, a separator and an
electrolytic solution, as the major constituent elements. The
electrolytic solution used is an organic solvent containing lithium
ions (hereunder also referred to as "nonaqueous electrolytic
solution").
<Positive Electrode>
The positive electrode of this embodiment has a positive electrode
power collector and a positive electrode active material layer
containing a positive electrode active material, formed on one or
both sides thereof.
The positive electrode of this embodiment preferably contains a
lithium compound as the positive electrode precursor, prior to
assembly of the nonaqueous lithium power storage element. As
mentioned above, during the step of assembling the nonaqueous
lithium power storage element of this embodiment, preferably the
negative electrode is predoped with lithium ion. The predoping
method for this embodiment is preferably application of a voltage
between the positive electrode precursor and negative electrode,
after the nonaqueous lithium power storage element has been
assembled using the positive electrode precursor containing a
lithium compound, the negative electrode, the separator and the
nonaqueous electrolytic solution. The lithium compound may be
included in any form in the positive electrode precursor and the
positive electrode. For example, the lithium compound may be
present between the positive electrode power collector and the
positive electrode active material layer, or it may be present on
the surface of the positive electrode active material layer. The
lithium compound is preferably contained in the positive electrode
active material layer formed on the positive electrode power
collector of the positive electrode precursor.
Throughout the present specification, "positive electrode
precursor" is defined as the positive electrode before the lithium
doping step, and "positive electrode" is defined as the positive
electrode after the lithium doping step.
In the solid .sup.7Li-NMR spectrum of the positive electrode active
material layer for this embodiment, the expression
1.04.ltoreq.b/a.ltoreq.5.56 is satisfied, where "a" is the peak
area from -40 ppm to 40 ppm, obtained by measurement with a
repeated latency of 10 seconds, and "b" is the peak area from -40
ppm to 40 ppm, obtained by measurement with a repeated latency of
3,000 seconds. The value of b/a is preferably
1.05.ltoreq.b/a.ltoreq.3.79, more preferably
1.09.ltoreq.b/a.ltoreq.3.32, even more preferably
1.14.ltoreq.b/a.ltoreq.2.86 and yet more preferably
1.18.ltoreq.b/a.ltoreq.1.93. The upper limit and lower limit may be
combined as desired.
By adjusting b/a to be within the specified range, it will be
possible to maintain a high input/output characteristic while
improving the high-load charge/discharge cycle characteristic.
While the principle is not completely understood, the following is
conjectured. The peak area "a" is believed to be for peaks derived
mainly from lithium ion intercalated in the positive electrode
active material and the adhering lithium-containing coating film,
and presumably it is a relative representation of the positive
electrode active material. On the other hand, the peak area "b" is
considered to be for peaks derived from lithium compounds that have
separated from the positive electrode active material, integrated
with the peak area "a". That is, b/a is considered to represent the
amount of lithium compound that has separated from the positive
electrode active material. The lithium compound that has separated
from the positive electrode active material can maintain a high
input/output characteristic without inhibiting electron conduction
or ion diffusion between the positive electrode active material.
Furthermore, the lithium compound adsorbs active products such as
fluorine ions that are generated during high-load charge/discharge
cycling, thereby improving the high-load charge/discharge cycle
characteristic. The term "separated" means a state in which, when
the positive electrode active material is an aggregate of activated
carbon particles, for example, lithium compound particles are
independently dispersed in its interior.
If b/a is 1.04 or greater, the amount of lithium compound with
respect to the positive electrode active material will be
sufficient, and therefore the lithium compound will adsorb active
products such as fluorine ions that are generated during high-load
charge/discharge cycling, thereby increasing the high-load
charge/discharge cycle characteristic. On the other hand, if b/a is
5.56 or smaller, the lithium compound will be able to maintain a
high input/output characteristic without inhibiting electron
conduction or ion diffusion between the positive electrode active
material.
Throughout the present specification, the area ratio b/a of the
peak area "a" in the spectral range of -40 ppm to 40 ppm, with a
repeated latency of 10 seconds, and the peak area "b" in the
spectral range of -40 ppm to 40 ppm, with a repeated latency of
3,000 seconds, in the solid .sup.7Li-NMR spectrum of the positive
electrode active material layer, can be calculated by the following
method.
The measuring apparatus used for solid .sup.7Li-NMR may be a
commercially available apparatus. The spectrum is measured by the
single pulse method in a room temperature environment, with a
magic-angle spinning rotational speed of 14.5 kHz and an
irradiation pulse width set to a 45.degree. pulse. Measurement is
performed with repeated latency of 10 seconds and 3,000 seconds, to
obtain a solid .sup.7Li-NMR spectrum. When obtaining the solid
.sup.7Li-NMR spectrum, the measuring conditions other than the
repeated latency, such as the number of scans and receiver gain,
are all identical. A 1 mol/L aqueous lithium chloride solution is
used as the shift reference, and the shift position measured
separately as an external standard is defined as 0 ppm. During
measurement of the 1 mol/L aqueous lithium chloride solution, the
single pulse method is used for spectral measurement with an
irradiation pulse width set to a 45.degree. pulse, without rotation
of the sample.
The peak areas "a" and "b" in a spectral range of -40 ppm to 40 ppm
are read off from the solid .sup.7Li-NMR spectrum of the positive
electrode active material layer obtained by the method described
above, and b/a is calculated.
For this embodiment, the amount of lithium in the positive
electrode, as calculated by the peak area from -40 ppm to 40 ppm in
the solid .sup.7Li-NMR spectrum of the positive electrode active
material layer of the positive electrode, is preferably 1 mmol/g to
30 mmol/g, more preferably 1.2 mmol/g to 28 mmol/g, even more
preferably 1.5 mmol/g to 26 mmol/g, yet more preferably 1.7 mmol/g
to 24 mmol/g and most preferably 2 mmol/g to 22 mmol/g, per unit
weight of the positive electrode active material layer.
The nonaqueous lithium power storage element of this embodiment has
charge/discharge cycle durability under high load while maintaining
a high input/output characteristic, by adjustment of the amount of
lithium in the positive electrode to a specific range. While the
principle is not completely understood, and it is not our intention
to be limited by theory, the following is conjectured. The amount
of lithium is thought to derive from the lithium-containing coating
film of the positive electrode active material layer. The
lithium-containing coating film, being internally polarized, has
high ionic conductivity, and therefore does not notably impair the
resistance even when formed in a large amount. Moreover, the
lithium-containing coating film can suppress oxidative
decomposition of the nonaqueous electrolytic solution. In addition,
since a lithium-containing coating film is stably present during
the charge/discharge process, compared to organic and inorganic
coating film components that do not contain lithium ion, the
coating film does not break even when charge/discharge cycling is
repeated a very large number of times, and there is minimal new
oxidative decomposition of the nonaqueous electrolytic solution.
Consequently, the power storage element can exhibit a high
high-load charge/discharge cycle characteristic.
If the amount of lithium in the positive electrode is at least 1
mmol/g per unit weight of the positive electrode active material
layer, the amount of lithium-containing coating film formed on the
positive electrode active material layer will be sufficient, thus
suppressing oxidative decomposition of the nonaqueous electrolytic
solution during charge/discharge cycling, and allowing a high
high-load charge/discharge cycle characteristic to be exhibited. If
the amount of lithium in the positive electrode is no greater than
30 mmol/g, increase in resistance due to the lithium-containing
coating film will be less likely to occur, and a high input/output
characteristic can be exhibited.
Throughout the present specification, the amount of lithium
obtained by the solid .sup.7Li-NMR spectrum of the positive
electrode active material layer can be calculated by the following
method.
The measuring apparatus used for solid .sup.7Li-NMR may be a
commercially available apparatus. The spectrum is measured by the
single pulse method in a room temperature environment, with a
magic-angle spinning rotational speed of 14.5 kHz and an
irradiation pulse width set to a 45.degree. pulse. The repeated
latency during the measurement is set for adequate measurement.
A 1 mol/L aqueous lithium chloride solution is used as the shift
reference, and the shift position measured separately as an
external standard is defined as 0 ppm. During measurement of the 1
mol/L aqueous lithium chloride solution, the single pulse method is
used for spectral measurement with an irradiation pulse width set
to a 45.degree. pulse, without rotation of the sample.
The obtained solid .sup.7Li-NMR spectrum for the positive electrode
active material layer obtained by the method described above is
used to determine the peak areas for components in the range of -40
ppm to 40 ppm. The peak areas are divided by the peak area for a 1
mol/L aqueous lithium chloride solution, with the same sample
height in the measuring rotor as during measurement of the positive
electrode active material layer, and further divided by the weight
of the positive electrode active material layer used for
measurement, to calculate the lithium amount in the positive
electrode. The weight of the positive electrode active material
layer is the weight of the positive electrode active material layer
including the coating film or deposits contained in the positive
electrode active material layer.
The positive electrode of this embodiment preferably comprises at
least one compound selected from the group consisting of compounds
represented by the following formulas (1) to (3), in an amount of
1.60.times.10.sup.-4 mol/g to 300.times.10.sup.-4 mol/g per unit
weight of the positive electrode active material layer.
[Chem. 4] LiX.sup.1--OR.sup.1O--X.sup.2Li (1) {In formula (1),
R.sup.1 is an alkylene group of 1 to 4 carbon atoms or a
halogenated alkylene group of 1 to 4 carbon atoms, and X.sup.1 and
X.sup.2 respectively and independently represent --(COO).sub.n
(where n is 0 or 1).} [Chem. 5]
LiX.sup.1--OR.sup.1O--X.sup.2R.sup.2 (2) {In formula (2), R.sup.1
is an alkylene group of 1 to 4 carbon atoms or a halogenated
alkylene group of 1 to 4 carbon atoms, R.sup.2 is hydrogen, an
alkyl group of 1 to 10 carbon atoms, a mono- or polyhydroxyalkyl
group of 1 to 10 carbon atoms, an alkenyl group of 2 to 10 carbon
atoms, a mono- or polyhydroxyalkenyl group of 2 to 10 carbon atoms,
a cycloalkyl group of 3 to 6 carbon atoms, or an aryl group, and
X.sup.1 and X.sup.2 respectively and independently represent
--(COO).sub.n (where n is 0 or 1).} [Chem. 6]
R.sup.2X.sup.1--OR.sup.1O--X.sup.2R.sup.3 (3) {In formula (3),
R.sup.1 is an alkylene group of 1 to 4 carbon atoms or a
halogenated alkylene group of 1 to 4 carbon atoms, R.sup.2 and
R.sup.3 respectively and independently represent hydrogen, an alkyl
group of 1 to 10 carbon atoms, a polyhydroxyalkyl group of 1 to 10
carbon atoms, an alkenyl group of 2 to 10 carbon atoms, a mono- or
polyhydroxyalkenyl group of 2 to 10 carbon atoms, a cycloalkyl
group of 3 to 6 carbon atoms or an aryl group, and X.sup.1 and
X.sup.2 respectively and independently represent --(COO).sub.n
(where n is 0 or 1).}
Particularly preferred as compounds of formula (1) are the
compounds represented by LiOC.sub.2H.sub.4OLi,
LiOC.sub.3H.sub.6OLi, LiOC.sub.2H.sub.4OCOOLi,
LiOCOOC.sub.3H.sub.6OLi, LiOCOOC.sub.2H.sub.4OCOOLi and
LiOCOOC.sub.3H.sub.6OCOOLi, for example, with no restriction to
these.
Particularly preferred as compounds of formula (2) are the
compounds represented by LiOC.sub.2H.sub.4OH, LiOC.sub.3H.sub.6OH,
LiOC.sub.2H.sub.4OCOOH, LiOC.sub.3H.sub.6OCOOH,
LiOCOOC.sub.2H.sub.4OCOOH, LiOCOOC.sub.3H.sub.6OCOOH,
LiOC.sub.2H.sub.4OCH.sub.3, LiOC.sub.3H.sub.6OCH.sub.3,
LiOC.sub.2H.sub.4OCOOCH.sub.3, LiOC.sub.3H.sub.6OCOOCH.sub.3,
LiOCOOC.sub.2H.sub.4OCOOCH.sub.3, LiOCOOC.sub.3H.sub.6OCOOCH.sub.3,
LiOC.sub.2H.sub.4OC.sub.2H.sub.5, LiOC.sub.3H.sub.6OC.sub.2H.sub.5,
LiOC.sub.2H.sub.4OCOOC.sub.2H.sub.5,
LiOC.sub.3H.sub.6OCOOC.sub.2H.sub.5,
LiOCOOC.sub.2H.sub.4OCOOC.sub.2H.sub.5 and
LiOCOOC.sub.3H.sub.6OCOOC.sub.2H.sub.5, for example, with no
restriction to these.
Particularly preferred as compounds of formula (3) are the
compounds represented by HOC.sub.2H.sub.4OH, HOC.sub.3H.sub.6OH,
HOC.sub.2H.sub.4OCOOH, HOC.sub.3H.sub.6OCOOH,
HOCOOC.sub.2H.sub.4OCOOH, HOCOOC.sub.3H.sub.6OCOOH,
HOC.sub.2H.sub.4OCH.sub.3, HOC.sub.3H.sub.6OCH.sub.3,
HOC.sub.2H.sub.4OCOOCH.sub.3, HOC.sub.3H.sub.6OCOOCH.sub.3,
HOCOOC.sub.2H.sub.4OCOOCH.sub.3, HOCOOC.sub.3H.sub.6OCOOCH.sub.3,
HOC.sub.2H.sub.4OC.sub.2H.sub.5, HOC.sub.3H.sub.6OC.sub.2H.sub.5,
HOC.sub.2H.sub.4OCOOC.sub.2H.sub.5,
HOC.sub.3H.sub.6OCOOC.sub.2H.sub.5,
HOCOOC.sub.2H.sub.4OCOOC.sub.2H.sub.5,
HOCOOC.sub.3H.sub.6OCOOC.sub.2H.sub.5,
CH.sub.3OC.sub.2H.sub.4OCH.sub.3, CH.sub.3OC.sub.3H.sub.6OCH.sub.3,
CH.sub.3OC.sub.2H.sub.4OCOOCH.sub.3,
CH.sub.3OC.sub.3H.sub.6OCOOCH.sub.3,
CH.sub.3OCOOC.sub.2H.sub.4OCOOCH.sub.3,
CH.sub.3OCOOC.sub.3H.sub.6OCOOCH.sub.3,
CH.sub.3OC.sub.2H.sub.4OC.sub.2H.sub.5,
CH.sub.3OC.sub.3H.sub.6OC.sub.2H.sub.5,
CH.sub.3OC.sub.2H.sub.4OCOOC.sub.2H.sub.5,
CH.sub.3OC.sub.3H.sub.6OCOOC.sub.2H.sub.5,
CH.sub.3OCOOC.sub.2H.sub.4OCOOC.sub.2H.sub.5,
CH.sub.3OCOOC.sub.3H.sub.6OCOOC.sub.2H.sub.5,
C.sub.2H.sub.5OC.sub.2H.sub.4OC.sub.2H.sub.5,
C.sub.2H.sub.5OC.sub.3H.sub.6OC.sub.2H.sub.5,
C.sub.2H.sub.5OC.sub.2H.sub.4OCOOC.sub.2H.sub.5,
C.sub.2H.sub.5OC.sub.3H.sub.6OCOOC.sub.2H.sub.5,
C.sub.2H.sub.5OCOOC.sub.2H.sub.4OCOOC.sub.2H.sub.5 and
C.sub.2H.sub.5OCOOC.sub.3H.sub.6OCOOC.sub.2H.sub.5, for example,
with no restriction to these.
For this embodiment, methods for adding a compound of formulas (1)
to (3) into the positive electrode active material layer include,
for example, a method of mixing a compound of formulas (1) to (3)
to the positive electrode active material layer; a method of
adsorbing a compound of formulas (1) to (3) onto the positive
electrode active material layer; and a method of electrochemically
depositing a compound of formulas (1) to (3) onto the positive
electrode active material layer.
As a method of adding a compound of formulas (1) to (3) to the
positive electrode active material layer, there is preferred a
method of adding a precursor that can decompose to produce such
compounds, into the nonaqueous electrolytic solution, and
decomposing the precursor during the step of fabricating the
nonaqueous lithium power storage element, to accumulate the
compound in the positive electrode active material layer.
Precursors that decompose to form compounds represented by formulas
(1) to (3) include one or more organic solvents selected from the
group consisting of ethylene carbonate, propylene carbonate,
butylene carbonate, vinylene carbonate and fluoroethylene
carbonate, with ethylene carbonate and propylene carbonate being
preferred.
The total amount of compounds of formulas (1) to (3) is preferably
1.60.times.10.sup.-4 mol/g or greater and more preferably
5.0.times.10.sup.-4 mol/g or greater, per unit weight of the
positive electrode active material layer. If the total amount of
compounds of formulas (1) to (3) is 1.60.times.10.sup.-4 mol/g or
greater per unit weight of the positive electrode active material
layer, then the nonaqueous electrolytic solution will be less
likely to come into contact with the positive electrode active
material, and oxidative decomposition of the nonaqueous
electrolytic solution can be more effectively suppressed.
The total amount of compounds of formulas (1) to (3) is preferably
no greater than 300.times.10.sup.-4 mol/g, more preferably no
greater than 150.times.10.sup.-4 mol/g and even more preferably no
greater than 100.times.10.sup.-4 mol/g, per unit weight of the
positive electrode active material layer. If the total amount of
compounds of formulas (1) to (3) is no greater than
300.times.10.sup.-4 mol/g per unit weight of the positive electrode
active material layer, diffusion of lithium ions will be less
inhibited and higher input/output characteristic can be
exhibited.
[Positive Electrode Active Material Layer]
The positive electrode active material layer contains a positive
electrode active material, but it may additionally contain optional
components such as a conductive filler, binder and dispersion
stabilizer, as necessary.
(Positive Electrode Active Material)
The positive electrode active material preferably contains a carbon
material. The carbon material is preferably carbon nanotubes, a
conductive polymer or a porous carbon material, and more preferably
activated carbon. The positive electrode active material may also
contain two or more different materials in admixture, and it may
even contain a material other than a carbon material such as, for
example, a complex oxide of lithium and a transition metal.
The content of the carbon material with respect to the total weight
of the positive electrode active material is preferably 50 weight %
or greater and more preferably 70 weight % or greater. The carbon
material content may be 100 weight %, but from the viewpoint of
obtaining a satisfactory effect by combined use with other
materials, it is preferably no greater than 90 weight % or no
greater than 80 weight %, for example.
When activated carbon is used as the positive electrode active
material, there are no particular restrictions on the type of
activated carbon or its starting material. However, preferably the
pores of the activated carbon are optimally controlled to obtain
both a high input/output characteristic and high energy density.
Specifically, if V.sub.1 (cc/g) is the mesopore volume due to pores
with diameters of 20 .ANG. to 500 .ANG. as calculated by the BJH
method, and V.sub.2 (cc/g) is the micropore volume due to pores
with diameters of smaller than 20 .ANG. as calculated by the MP
method, then:
(1) in order to obtain a high input/output characteristic,
activated carbon satisfying 0.3<V.sub.1.ltoreq.0.8 and
0.5.ltoreq.V.sub.2.ltoreq.1.0 and exhibiting a specific surface
area of 1,500 m.sup.2/g to 3,000 m.sup.2/g as measured by the BET
method (hereunder also referred to as "activated carbon 1") is
preferred, and
(2) in order to obtain high energy density, activated carbon
satisfying 0.8<V.sub.1.ltoreq.2.5 and 0.8<V.sub.2.ltoreq.3.0
and exhibiting a specific surface area of 2,300 m.sup.2/g to 4,000
m.sup.2/g as measured by the BET method (hereunder also referred to
as "activated carbon 2"), is preferred.
The (1) activated carbon 1 and (2) activated carbon 2 will now be
described.
(Activated Carbon 1)
The mesopore volume V.sub.1 of activated carbon 1 is preferably a
value larger than 0.3 cc/g, from the viewpoint of a greater
input/output characteristic when the positive electrode material
has been incorporated into a nonaqueous lithium power storage
element. On the other hand, V.sub.1 for activated carbon 1 is also
preferably no greater than 0.8 cc/g from the viewpoint of
minimizing reduction in the bulk density of the positive electrode.
V.sub.1 for activated carbon 1 is more preferably 0.35 cc/g to 0.7
cc/g and even more preferably 0.4 cc/g to 0.6 cc/g.
The micropore volume V.sub.2 of activated carbon 1 is preferably
0.5 cc/g or greater in order to increase the specific surface area
of the activated carbon and increase capacitance. On the other
hand, from the viewpoint of minimizing the bulk of the activated
carbon, increasing the density as an electrode and increasing the
capacitance per unit volume, V.sub.2 for activated carbon 1 is
preferably no larger than 1.0 cc/g. V.sub.2 for activated carbon 1
is more preferably 0.6 cc/g to 1.0 cc/g and even more preferably
0.8 cc/g to 1.0 cc/g.
The ratio of the mesopore volume V.sub.1 to the micropore volume
V.sub.2 for activated carbon 1 (V.sub.1/V.sub.2) is preferably in
the range of 0.3.ltoreq.V.sub.1/V.sub.2.ltoreq.0.9. That is,
V.sub.1/V.sub.2 for activated carbon 1 is preferably 0.3 or greater
from the viewpoint of increasing the ratio of the mesopore volume
to the micropore volume to a degree allowing reduction in the
output characteristic to be minimized while maintaining high
capacitance. On the other hand, V.sub.1/V.sub.2 for activated
carbon 1 is preferably no greater than 0.9 from the viewpoint of
increasing the ratio of the micropore volume with respect to the
mesopore volume, to a degree allowing a high output characteristic
to be maintained while minimizing reduction in capacitance. The
range of V.sub.1/V.sub.2 for activated carbon 1 is more preferably
0.4.ltoreq.V.sub.1/V.sub.2.ltoreq.0.7 and even more preferably
0.55.ltoreq.V.sub.1/V.sub.2.ltoreq.0.7.
The mean pore size of activated carbon 1 is preferably 17 .ANG. or
greater, more preferably 18 .ANG. or greater and even more
preferably 20 .ANG. or greater, from the viewpoint of increasing
the output of the obtained nonaqueous lithium power storage
element. From the viewpoint of increasing capacitance, the mean
pore size of activated carbon 1 is preferably no greater than 25
.ANG..
The BET specific surface area of activated carbon 1 is preferably
1,500 m.sup.2/g to 3,000 m.sup.2/g, and more preferably 1,500
m.sup.2/g to 2,500 m.sup.2/g. If the BET specific surface area of
activated carbon 1 is 1,500 m.sup.2/g or greater it will be easier
to obtain satisfactory energy density, while if the BET specific
surface area of activated carbon 1 is 3,000 m.sup.2/g or lower
there will be no need to add large amounts of a binder to maintain
the strength of the electrode, and therefore the performance per
volume of the electrode will be higher.
The activated carbon 1 having such features can be obtained, for
example, using the starting material and treatment method described
below.
For this embodiment, the carbon source to be used as the starting
material for activated carbon 1 is not particularly restricted.
Examples of carbon sources for activated carbon 1 include
plant-based starting materials such as wood, wood dust, coconut
shell, by-products of pulp production, bagasse and molasses;
fossil-based starting materials such as peat, lignite, brown coal,
bituminous coal, anthracite, petroleum distillation residue
components, petroleum pitch, coke and coal tar; various synthetic
resins such as phenol resin, vinyl chloride resin, vinyl acetate
resin, melamine resin, urea resin, resorcinol resin, celluloid,
epoxy resin, polyurethane resin, polyester resin and polyamide
resin; synthetic rubbers such as polybutylene, polybutadiene and
polychloroprene; as well as synthetic wood or synthetic pulp
materials, and carbides of the foregoing. From the viewpoint of
suitability for mass-production and of cost, the starting materials
preferred among these are plant-based starting materials such as
coconut shell and wood dust, and their carbides, with coconut shell
carbides being particularly preferred.
The system used for carbonization and activation from these
starting materials to produce the activated carbon 1 may be a known
system such as, for example, a fixed bed system, moving bed system,
fluidized bed system, slurry system or rotary kiln system.
The carbonization method for these starting materials is a method
in which an inert gas such as nitrogen, carbon dioxide, helium,
argon, xenon, neon, carbon monoxide or exhaust gas, or a mixed gas
composed mainly of such inert gases with other gases, is used for
calcination at 400 to 700.degree. C. and preferably 450 to
600.degree. C., over a period of about 30 minutes to 10 hours.
The activation method for a carbide obtained by the carbonization
method is preferably a gas activation method in which an activating
gas such as water vapor, carbon dioxide or oxygen is used for
calcination. A method using water vapor or carbon dioxide as the
activating gas is more preferred.
In this activation method, the activating gas is supplied at a rate
of 0.5 to 3.0 kg/h and preferably 0.7 to 2.0 kg/h, while the
carbide is raised to 800 to 1,000.degree. C. for 3 to 12 hours,
preferably 5 to 11 hours and more preferably 6 to 10 hours, for
activation.
The carbide may be subjected to a primary activation before
activation treatment of the carbide. In the primary activation, a
method of calcinating the carbon material at a temperature of below
900.degree. C. using an activating gas such as water vapor, carbon
dioxide or oxygen for gas activation, is usually preferred.
By appropriate combinations for the calcination temperature and
calcination time for the carbonization method, and the activating
gas supply rate, temperature-elevating rate and maximum activation
temperature in the activation method, it is possible to produce
activated carbon 1 having the features described above, which is
preferred for this embodiment.
The mean particle diameter of the activated carbon 1 is preferably
2 to 20 .mu.m. If the mean particle diameter of the activated
carbon 1 is 2 .mu.m or greater, the capacitance per electrode
volume will tend to be higher due to the higher density of the
active material layer. If the mean particle diameter of the
activated carbon 1 is small, the durability may be reduced, but the
durability is unlikely to be reduced if the mean particle diameter
is 2 .mu.m or greater. A mean particle diameter of the activated
carbon 1 of no larger than 20 .mu.m will tend to be more suitable
for high-speed charge/discharge. The mean particle diameter of
activated carbon 1 is more preferably 2 to 15 .mu.m and even more
preferably 3 to 10 .mu.m.
(Activated Carbon 2)
The mesopore volume V.sub.1 of activated carbon 2 is preferably a
value larger than 0.8 cc/g, from the viewpoint of a greater
input/output characteristic when the positive electrode material
has been incorporated into a nonaqueous lithium power storage
element. On the other hand, it is preferably no greater than 2.5
cc/g from the viewpoint of minimizing reduction in the capacitance
of the nonaqueous lithium power storage element. V for activated
carbon 2 is more preferably 1.00 cc/g to 2.0 cc/g and even more
preferably 1.2 cc/g to 1.8 cc/g.
The micropore volume V.sub.2 of activated carbon 2 is preferably a
value larger than 0.8 cc/g in order to increase the specific
surface area of the activated carbon and increase capacitance. From
the viewpoint of increasing the density of the activated carbon as
an electrode and increasing the capacitance per unit volume, the
V.sub.2 value of activated carbon 2 is preferably no greater than
3.0 cc/g, more preferably greater than 1.0 cc/g and no greater than
2.5 cc/g, and even more preferably 1.5 cc/g to 2.5 cc/g.
Activated carbon 2 having the mesopore volume and micropore volume
described above has a higher BET specific surface area than
activated carbon used in conventional electrical double layer
capacitors or lithium ion capacitors. The specific value of the BET
specific surface area of activated carbon 2 is preferably 2,300
m.sup.2/g to 4,000 m.sup.2/g. The lower limit for the BET specific
surface area is more preferably 3,000 m.sup.2/g or greater and even
more preferably 3,200 m.sup.2/g or greater. The upper limit for the
BET specific surface area is more preferably no greater than 3,800
m.sup.2/g. If the BET specific surface area of activated carbon 2
is 2,300 m.sup.2/g or greater it will be easier to obtain
satisfactory energy density, and if the BET specific surface area
of activated carbon 2 is 4,000 m.sup.2/g or lower there will be no
need to add large amounts of a binder to maintain the strength of
the electrode, and therefore the performance per volume of the
electrode will tend to be higher.
Activated carbon 2 having such features can be obtained, for
example, using the starting material and treatment method described
below.
The carbon source used as the starting material for activated
carbon 2 is not particularly restricted so long as it is a carbon
source commonly used as a starting material for activated carbon,
and examples include plant-based starting materials such as wood,
wood dust and coconut shell; petroleum-based starting materials
such as petroleum pitch and coke; and various synthetic resins such
as phenol resins, furan resins, vinyl chloride resins, vinyl
acetate resins, melamine resins, urea resins and resorcinol resins.
Of these starting materials, phenol resins and furan resins are
especially preferred, being suitable for fabrication of activated
carbon 2 with a high specific surface area.
The system used for carbonization of these starting materials, or
the heating method during activation treatment, may be a known
system such as, for example, a fixed bed system, moving bed system,
fluidized bed system, slurry system or rotary kiln system. The
atmosphere during heating is an inert gas such as nitrogen, carbon
dioxide, helium or argon, or a mixed gas composed mainly of such
inert gases in admixture with other gases. The carbonization
temperature is preferably 400 to 700.degree. C. The lower limit for
the carbonization temperature is preferably 450.degree. C. or
higher and more preferably 500.degree. C. or higher. The upper
limit for the carbonization temperature is preferably no higher
than 650.degree. C. The carbonization time is preferably
calcination of the starting materials for about 0.5 to 10
hours.
The activation method for the carbide after carbonization may be a
gas activation method in which calcination is accomplished using an
activating gas such as water vapor, carbon dioxide or oxygen, or an
alkali metal activation method in which heat treatment is carried
out after mixture with an alkali metal compound. An alkali metal
activation method is preferred to produce activated carbon with a
high specific surface area.
In this activation method, preferably a carbide and an alkali metal
compound such as KOH or NaOH are mixed so that the weight ratio is
1:.gtoreq.1 (the amount of the alkali metal compound being equal to
or greater than the amount of the carbide), after which heat
treatment is carried out in a range of 600 to 900.degree. C. and
preferably 650.degree. C. to 850.degree. C. for 0.5 to 5 hours
under an inert gas atmosphere, and then the alkali metal compound
is subjected to cleaning removal with an acid or water, and drying
is performed.
A greater amount of alkali metal compound with respect to carbide
will tend to increase the mesopore volume, with a drastic increase
in the pore volume near a weight ratio of 1:3.5, and therefore the
amount of alkali metal compound is preferably larger than a
carbide:alkali metal compound weight ratio of 1:3, while also being
preferably 1:.ltoreq.5.5. Although the pore volume increases as the
alkali metal compound increases with respect to the carbide, it is
preferably 1:.ltoreq.5.5 in consideration of the efficiency of
subsequent treatment procedures such as washing.
In order to increase the micropore volume and not increase the
mesopore volume, the amount of carbide may be increased during
activation, and mixed with KOH. In order to increase both the
micropore volume and mesopore volume, a large amount of KOH may be
used. In order to increase mainly the mesopore volume,
steam-activation is preferably carried out after alkaline
activation treatment.
The mean particle diameter of activated carbon 2 is preferably 1
.mu.m to 30 .mu.m, more preferably 2 .mu.m to 20 .mu.m and even
more preferably 3 .mu.m to 10 .mu.m.
(Aspect Using Activated Carbon)
When activated carbon is to be used for the positive electrode
active material, activated carbons 1 and 2 may each be a single
type of activated carbon or a mixture of two or more different
types of activated carbon, such that the mixture as a whole
exhibits the characteristic values described above.
Either of activated carbon 1 or 2 may be selected for use, or both
may be used in admixture.
The positive electrode active material may also include materials
other than activated carbons 1 and 2, such as activated carbon
without the specified V.sub.1 and/or V.sub.2 values, or materials
other than activated carbon, such as complex oxides of lithium and
transition metals. In the exemplary aspect, the content of
activated carbon 1, or the content of activated carbon 2, or the
total content of activated carbons 1 and 2, are preferably greater
than 50 weight %, more preferably 70 weight % or greater, even more
preferably 90 weight % or greater and yet more preferably 100
weight %, of the total positive electrode active material.
The content ratio of the positive electrode active material in the
positive electrode is preferably 35 weight % to 95 weight % based
on the total weight of the positive electrode active material layer
in the positive electrode precursor. The lower limit for the
content ratio of the positive electrode active material is more
preferably 45 weight % or greater and even more preferably 55
weight % or greater. On the other hand, the upper limit for the
content ratio of the positive electrode active material is more
preferably no greater than 90 weight % and even more preferably no
greater than 85 weight %. A suitable charge/discharge
characteristic is exhibited by adjusting the content ratio of the
positive electrode active material to within this range.
(Lithium Compound)
Through the present specification, "lithium compound" refers to a
lithium compound that is not the positive electrode active material
and not a compound of formulas (1) to (3).
The lithium compound may be one or more selected from the group
consisting of lithium carbonate, lithium oxide, lithium hydroxide,
lithium fluoride, lithium chloride, lithium bromide, lithium
iodide, lithium nitride, lithium oxalate and lithium acetate, that
can decompose at the positive electrode in the lithium doping step
described below, releasing lithium ion. Preferred among these are
lithium carbonate, lithium oxide and lithium hydroxide, with
lithium carbonate being more preferred, from the viewpoint of being
handleable in air and having low hygroscopicity. Such lithium
compounds decompose upon application of a voltage, to function as a
dopant source for lithium doping in the negative electrode, while
also forming a satisfactory coating film on the positive electrode
active material layer, and thus allowing a positive electrode to be
formed that exhibits a high high-load charge/discharge cycle
characteristic.
The lithium compound is preferably in particulate form. The mean
particle diameter of the particulate lithium compound is preferably
0.1 .mu.m to 10 .mu.m. If the mean particle diameter of the lithium
compound is 0.1 .mu.m or larger, the volume of pores remaining
after oxidation reaction of the lithium compound at the positive
electrode will be sufficiently large to hold the nonaqueous
electrolytic solution, and the high-load charge/discharge cycle
characteristic will therefore be improved. If the mean particle
diameter of the particulate lithium compound is no larger than 10
.mu.m, the surface area of the lithium compound will not be
excessively reduced, and the speed of the oxidation reaction of the
lithium compound can be ensured.
Various methods may be used for micronization of the lithium
compound. For example, a pulverizer such as a ball mill, bead mill,
ring mill, jet mill or rod mill may be used.
The content ratio of the lithium compound in the positive electrode
is preferably 1 weight % to 20 weight % and more preferably 2
weight % to 18 weight %, based on the total weight of the positive
electrode active material layer at the positive electrode. If the
content ratio of the lithium compound in the positive electrode is
1 weight % or greater, a sufficient amount of lithium compound will
be present to trap active products such as fluorine ions formed in
the high-load charge/discharge cycling, and therefore the high-load
charge/discharge cycle characteristic will be improved. If the
content ratio of the lithium compound in the positive electrode is
no greater than 20 weight %, it will be possible to increase the
energy density of the nonaqueous lithium power storage element.
The content ratio of the lithium compound in the positive electrode
precursor is preferably 10 weight % to 60 weight % and more
preferably 20 weight % to 50 weight %, based on the total weight of
the positive electrode active material layer in the positive
electrode precursor. By adjusting the content ratio of the lithium
compound in the positive electrode precursor to be 10 weight % to
60 weight %, a suitable function is exhibited as a dopant source in
the negative electrode, a suitable degree of porosity can be
imparted to the positive electrode and a satisfactory coating film
can be formed, thereby allowing a nonaqueous lithium power storage
element with an excellent high-load charge/discharge cycle
characteristic to be obtained.
[Method of Identifying Lithium Compound in Positive Electrode]
The method of identifying a lithium compound in the positive
electrode is not particularly restricted, and it may be
identification by the following methods, for example. For
identification of a lithium compound, it is preferred to carry out
the identification by combining the different analysis methods
described below.
For measurement by SEM-EDX, Raman spectroscopy or XPS described
below, preferably the nonaqueous lithium power storage element is
disassembled in an argon box, the positive electrode is removed,
and measurement is performed after washing the electrolytic
adhering to the positive electrode surface. The solvent used to
wash the positive electrode only needs to wash off the electrolyte
adhering to the positive electrode surface, and a carbonate solvent
such as dimethyl carbonate, ethylmethyl carbonate or diethyl
carbonate may be suitably used. The washing method may be, for
example, immersion of the positive electrode for 10 minutes or
longer in a diethyl carbonate solvent in an amount of 50 to 100
times the weight of the positive electrode, and subsequent
reimmersion of the positive electrode after exchange of the
solvent. The positive electrode is then removed from the diethyl
carbonate and vacuum dried, and then subjected to SEM-EDX, Raman
spectroscopy and XPS analysis. The vacuum drying conditions are
conditions such that the diethyl carbonate residue in the positive
electrode is no greater than 1 weight % under the conditions of a
temperature of 0 to 200.degree. C., a pressure of 0 to 20 kPa and a
time of 1 to 40 hours. The diethyl carbonate residue can be
quantified by GC/MS measurement of water after distilled water
washing and liquid volume adjustment, based on a predrawn
calibration curve.
In ion chromatography, described below, the water after washing of
the positive electrode with distilled water is analyzed to allow
identification of the anion.
When the lithium compound cannot be identified by this analysis
method, solid .sup.7Li-NMR, XRD (X-ray diffraction), TOF-SIMS
(Time-Of-Flight Secondary Ion Mass Spectrometry), AES (Auger
Electron Spectroscopy), TPD/MS (Temperature Programmed
Desorption/Mass Spectrometry), DSC (Differential Scanning
Calorimetry) or the like may be used as an alternative analysis
method to identify the lithium compound.
(Energy Dispersive X-Ray Analysis (SEM-EDX))
The oxygen-containing lithium compound and positive electrode
active material can be discriminated by oxygen mapping with an
SEM-EDX image of the positive electrode surface measured at an
observational magnification of 1,000-4,000.times.. The SEM-EDX
image can be measured with an acceleration voltage of 10 kV, an
emission current of 10 .mu.A and a measuring pixel count of
256.times.256 pixels, and a number of scans of 50. In order to
prevent electrification of the sample, the sample may be surface
treated with gold, platinum, osmium or the like by a method such as
vacuum vapor deposition or sputtering. For the SEM-EDX image
measuring method, preferably the luminance and contrast are
adjusted so that the brightness has no pixel reaching the maximum
luminance, and the mean value of the brightness falls within the
range of 40% to 60% of the luminance. In the obtained oxygen
mapping, particles containing a bright portion equal to 50% or more
of the area thereof when binarized based on the mean value of
brightness with respect to the resulting oxygen mapping were
considered to be the lithium compound.
(Raman Spectroscopy)
The lithium compound comprising carbonate ion, and the positive
electrode active material can be discriminated by Raman imaging of
the positive electrode surface measured at an observational
magnification of 1,000-4,000x. The measuring conditions may be, for
example, an excitation light of 532 nm, an excitation light
intensity of 1%, 50-fold long working of objective lens, a
diffraction grating of 1,800 gr/mm, point scanning as the mapping
system (slit: 65 mm, binning: 5 pix), a 1 mm step, an exposure time
per point of 3 seconds, a number of scans of 1, and a noise filter.
For the measured Raman spectrum, a straight baseline is set in the
range of 1,071 to 1,104 cm.sup.-1, a value positive from the
baseline is considered a carbonate ion peak, and the area is
calculated and the frequency is integrated. The frequency with
respect to the carbonate ion peak area with the noise component
approximated by a Gaussian function is subtracted from the
carbonate ion frequency distribution.
(X-Ray Photoelectron Spectroscopy (XPS))
The electronic state of lithium can be analyzed by XPS to
discriminate the bonded state of the lithium. The measuring
conditions may be, for example, monochromatized AlK.alpha. as the
X-ray source, an X-ray beam diameter of 100 .mu.m.phi. (25 W, 15
kV), narrow scan for path energy (58.70 eV), with charge
neutralization, narrow scan for sweeping: 10 times (carbon,
oxygen), 20 times (fluorine), 30 times (phosphorus), 40 times
(lithium), 50 times (silicon), narrow scan for energy step: 0.25
eV. The surface of the positive electrode is preferably cleaned by
sputtering before XPS measurement. As the sputtering conditions,
cleaning of the positive electrode surface may be carried out, for
example, with an acceleration voltage of 1.0 kV, and 1 minute in a
range of 2 mm.times.2 mm (1.25 nm/min as SiO.sub.2).
In the obtained XPS spectrum, the following assignments may be
made: a peak having Li1s bonding energy of 50 to 54 eV as a
LiO.sub.2 or Li--C bond, a peak of 55 to 60 eV as LiF,
Li.sub.2CO.sub.3, Li.sub.xPO.sub.yF.sub.z (where x, y and z are
each an integer of 1 to 6); a peak having C1s bonding energy of 285
eV as a C--C bond, a peak of 286 eV as a C--O bond, a peak of 288
eV as COO, a peak of 290 to 292 eV as CO.sub.3.sup.2- and C--F
bonds; a peak having O1s bonding energy of 527 to 530 eV as
O.sup.2- (Li.sub.2O), a peak of 531 to 532 eV as CO, CO.sub.3, OH,
PO.sub.x (where x is an integer of 1 to 4), SiO.sub.x (where x is
an integer of 1 to 4), a peak of 533 eV as C--O or SiO.sub.x (where
x is an integer of 1 to 4); a peak having F1s bonding energy of 685
eV as LiF, a peak of 687 eV as a C--F bond, Li.sub.xPO.sub.yF.sub.z
(where x, y and z are integers of 1 to 6), PF.sub.6.sup.-; and for
P2p bonding energy, a peak of 133 eV as PO.sub.x (where x is an
integer of 1 to 4), a peak of 134 to 136 eV as PF.sub.x (where x is
an integer of 1 to 6); a peak having Si2p bonding energy of 99 eV
as Si or silicide, a peak of 101 to 107 eV as Si.sub.xO.sub.y
(where x and y are each an arbitrary integer).
When peaks overlap in the obtained spectrum, the spectrum is
preferably assigned upon separating the peaks with the assumption
of a Gaussian function or Lorentz function. The lithium compound
that is present can be identified based on the obtained results of
measuring the electronic state, and the existing element ratio.
(Ion Chromatography)
Anion species eluted in water can be identified by washing the
positive electrode with distilled water and analyzing the water
after washing, by ion chromatography. The columns used may be an
ion-exchange type, ion exclusion type and reversed-phase ion pair
type. The detector used may be an electric conductivity detector,
ultraviolet-visible absorption intensity detector or
electrochemical detector, and a suppressor system with a suppressor
installed before the detector, or a non-suppressor system without
installation of a suppressor, using a solution with low electric
conductivity as the eluent, may be used. Since measurement can also
be carried out by combining a mass spectrometer or charged particle
detector with the detector, it is preferred to combine an
appropriate column and detector, depending on the lithium compound
identified from the results of analysis by SEM-EDX, Raman
spectroscopy or XPS.
The sample holding time will depend on the conditions such as the
column and eluent used and is the same for each ion species
component, while the size of the peak response differs for each ion
species but is proportional to the concentration of the ion
species. By premeasuring a standard solution of known concentration
with ensured traceability, it is possible to qualitatively and
quantitatively analyze the ion species components.
[Method of Quantifying Lithium Compound]
A method of quantifying the lithium compound in the positive
electrode will now be described.
The positive electrode may be washed with an organic solvent and
subsequently washed with distilled water, and the lithium compound
quantified from the change in positive electrode weight before and
after the washing with distilled water. The area of the positive
electrode to be measured is not particularly restricted, but from
the viewpoint of reducing measurement variation it is preferably 5
cm.sup.2 to 200 cm.sup.2 and more preferably 25 cm.sup.2 to 150
cm.sup.2. Measurement reproducibility can be ensured if the area is
at least 5 cm.sup.2. The handleability of the sample will be
excellent if the area is no greater than 200 cm.sup.2. Washing with
an organic solvent is sufficient if it can remove decomposition
products of the nonaqueous electrolytic solution that have
accumulated on the positive electrode surface, and therefore while
the organic solvent is not particularly restricted, elution of the
lithium compound can be suppressed by using an organic solvent with
a solubility of no greater than 2% for the lithium compound, and it
is therefore preferred. For example, a polar solvent such as
methanol or acetone may be used.
The method of washing the positive electrode is thorough immersion
of the positive electrode for 3 days or longer in a methanol
solution at a 50- to 100-fold amount with respect to the weight of
the positive electrode. During the procedure, certain measures are
preferred such as capping of the vessel so that the methanol does
not volatilize off. The positive electrode is then removed from the
methanol and subjected to vacuum drying (under conditions such that
the methanol residue in the positive electrode is no greater than 1
weight % with a temperature of 100 to 200.degree. C., a pressure of
0 to 10 kPa and a time of 5 to 20 hours. The methanol residue can
be quantified by GC/MS measurement of water after distilled water
washing, based on a predrawn calibration curve, as described
below.), and the weight of the positive electrode at that time is
recorded as M.sub.0 (g). The positive electrode is thoroughly
immersed for 3 days or longer in distilled water at a 100-fold
amount (100 M.sub.0 (g)) with respect to the weight of the positive
electrode. During the procedure, certain measures are preferred
such as capping of the vessel so that the distilled water does not
volatilize off. After immersion for 3 days or longer, the positive
electrode is removed from the distilled water (for the
aforementioned ion chromatography measurement, the liquid volume is
adjusted so that the amount of distilled water is 100 M.sub.0 (g)),
and vacuum drying is performed in the same manner as for the
methanol washing described above. The weight of the positive
electrode at this time is recorded as M.sub.1 (g), and then the
positive electrode active material layer is removed from the power
collector using a spatula, brush, bristles or the like, for
measurement of the weight of the obtained positive electrode power
collector. If the weight of the obtained positive electrode power
collector is represented as M.sub.2 (g), the ratio Z (weight %) of
the lithium compound in the positive electrode can be calculated by
the following formula.
Z=100.times.[1-(M.sub.1-M.sub.2)/(M.sub.0-M.sub.2)] [Mean Particle
Diameter of Lithium Compound and Positive Electrode Active
Material]
Preferably, the expression 0.1 .mu.m.ltoreq.X.sub.1.ltoreq.10 .mu.m
is satisfied where X.sub.1 is the mean particle diameter of the
lithium compound and the expressions 2
.mu.m.ltoreq.Y.sub.1.ltoreq.20 .mu.m and X.sub.1<Y.sub.1 are
satisfied, where Y.sub.1 is the mean particle diameter of the
positive electrode active material. More preferably, X.sub.1
satisfies 0.5 .mu.m.ltoreq.X.sub.1.ltoreq.5 .mu.m, and Y.sub.1
satisfies 3 .mu.m.ltoreq.Y.sub.1.ltoreq.10 .mu.m. If X.sub.1 is 0.1
.mu.m or greater, it will be possible to have lithium compound
remaining in the positive electrode after predoping of lithium, and
therefore the high-load charge/discharge cycle characteristic will
be increased by adsorption of fluorine ions produced by high-load
charge/discharge cycling. If X.sub.1 is no greater than 10 .mu.m,
on the other hand, the reaction area with the fluorine ions
generated by the high-load charge/discharge cycling will increase,
thus allowing the fluorine ions to be adsorbed more efficiently. If
Y.sub.1 is 2 .mu.m or greater, it will be possible to ensure
electron conductivity between the positive electrode active
materials. If Y.sub.1 is no greater than 20 .mu.m, on the other
hand, the reaction area with the electrolytic ion will increase,
allowing a high input/output characteristic to be obtained. If
X.sub.1<Y.sub.1, then the lithium compound will fill in the gaps
formed between the positive electrode active material, thus
allowing the electron conductivity between the positive electrode
active material to be ensured while increasing the energy
density.
The method of measuring X.sub.1 and Y.sub.1 is not particularly
restricted, and they may be calculated from an SEM image and
SEM-EDX image of the positive electrode cross-section. The method
of forming the positive electrode cross-section may employ BIB
processing in which an Ar beam is irradiated from above the
positive electrode, and a smooth cross-section is created along the
edges of a masking shield set directly above the sample. When the
positive electrode comprises lithium carbonate, the carbonate ion
distribution can be determined by measurement with Raman imaging of
the positive electrode cross-section.
[Method of Discriminating Lithium Compound and Positive Electrode
Active Material]
The lithium compound and positive electrode active material can be
discriminated by oxygen mapping with an SEM-EDX image of the
positive electrode cross-section measured at an observational
magnification of 1000-4000x. For the SEM-EDX image measuring
method, preferably the luminance and contrast are adjusted so that
the brightness has no pixel reaching the maximum luminance, and the
mean value of the brightness is a luminance in the range of 40% to
60%. In the obtained oxygen mapping, particles containing a bright
portion equal to 50% or more of the area thereof when binarized
based on the mean value of brightness with respect to the resulting
oxygen mapping were considered to be lithium compound.
[Method of Calculating X.sub.1 and Y.sub.1]
X.sub.1 and Y.sub.1 can be determined by analysis of an image
obtained from positive electrode cross-sectional SEM-EDX, measured
in the same visual field as the positive electrode cross-sectional
SEM mentioned above. The cross-sectional area S is determined for
all of the particles X and Y observed in the cross-sectional SEM
image, X being lithium compound particles discriminated in the SEM
image of the positive electrode cross-section, and Y being the
other particles which are particles of the positive electrode
active material, and the particle diameter d is determined by the
following formula (where .pi. is the circular constant).
d=2.times.(S/.pi.).sup.1/2
Each obtained particle diameter d is used to determine the
volume-average particle diameters X.sub.0 and Y.sub.0, by the
following formula.
X.sub.0(Y.sub.0)=.SIGMA.[4/3.pi..times.(d/2).sup.3.times.d]/.SIGMA.[4/3.p-
i..times.(d/2).sup.3]
Measurement is performed at five or more locations varying the
visual field of the positive electrode cross-section, and the mean
values of X.sub.0 and Y.sub.0 are recorded as the mean particle
diameters X.sub.1 and Y.sub.1.
(Optional Components)
If necessary, the positive electrode active material layer of this
embodiment may also contain optional components such as a
conductive filler, binder and dispersion stabilizer, in addition to
the positive electrode active material and lithium compound.
The conductive filler is not particularly restricted, and for
example, acetylene black, Ketchen black, vapor grown carbon fibers,
graphite, carbon nanotubes, and mixtures thereof, may be used. The
amount of conductive filler used is preferably 0 parts by weight to
30 parts by weight, more preferably 0 parts by weight to 20 parts
by weight and even more preferably 1 part by weight to 15 parts by
weight, with respect to 100 parts by weight of the positive
electrode active material. If the amount of conductive filler used
is no greater than 30 parts by weight, the content ratio of the
positive electrode active material in the positive electrode active
material layer will be increased, allowing the energy density per
volume of the positive electrode active material layer to be
ensured.
The binder is not particularly restricted, and for example, PVdF
(polyvinylidene fluoride), PTFE (polytetrafluoroethylene),
polyimide, latex, styrene-butadiene copolymer, fluorine rubber or
an acrylic copolymer may be used. The amount of binder used is
preferably 1 part by weight to 30 parts by weight, more preferably
3 parts by weight to 27 parts by weight and even more preferably 5
parts by weight to 25 parts by weight, with respect to 100 parts by
weight of the positive electrode active material. If the amount of
binder used is 1 part by weight or greater, adequate electrode
strength will be exhibited. If the amount of binder used is no
greater than 30 parts by weight, on the other hand, a high
input/output characteristic will be exhibited without inhibiting
movement or diffusion of ions in and from the positive electrode
active material.
The dispersion stabilizer is not particularly restricted, and for
example, PVP (polyvinylpyrrolidone), PVA (polyvinyl alcohol) or
cellulose derivatives may be used. The amount of dispersion
stabilizer used is preferably 0 parts by weight to 10 parts by
weight, with respect to 100 parts by weight of the positive
electrode active material. If the amount of dispersion stabilizer
used is no greater than 10 parts by weight, on the other hand, a
high input/output characteristic will be exhibited without
inhibiting movement or diffusion of ions in and from the positive
electrode active material.
[Positive Electrode Power Collector]
The material composing the positive electrode power collector of
this embodiment is not particularly restricted so long as it is a
material with high electron conductivity, and resistance to
degradation by elution into the nonaqueous electrolytic solution or
reaction with the electrolyte or ion, but a metal foil is
preferred. The positive electrode power collector in the nonaqueous
lithium power storage element of this embodiment is most preferably
an aluminum foil.
The metal foil may be a common metal foil without ruggedness or
through-holes, or it may be a metal foil having ruggedness formed
by embossing, chemical etching, electrolytic deposition or
blasting, or it may be a metal foil having through-holes, such as
an expanded metal, punching metal or etching foil.
Among these, the positive electrode power collector of this
embodiment is preferably a metal foil without through-holes. Having
no through-holes is cheaper in terms of production cost and
facilitates thin-film formation, and can thus contribute to higher
energy density, while also lowering the power collector resistance
and allowing a high input/output characteristic to be obtained.
The thickness of the positive electrode power collector is not
particularly restricted so long as it allows the shape and strength
of the positive electrode to be maintained, but 1 to 100 .mu.m, for
example, is preferred.
[Production of Positive Electrode Precursor]
According to this embodiment, the positive electrode precursor that
is to be the positive electrode of the nonaqueous lithium power
storage element can be produced by a known production technique for
electrodes for lithium ion batteries or electrical double layer
capacitors. For example, the positive electrode active material and
lithium compound, as well as the other optional components that are
used as necessary, may be dispersed and dissolved in water or an
organic solvent to prepare a slurry-like coating solution, and the
coating solution coated onto one or both sides of a positive
electrode power collector to form a coating film, which is dried to
obtain a positive electrode precursor. The obtained positive
electrode precursor may also be pressed to adjust the film
thickness or bulk density of the positive electrode active material
layer. An alternative method may also be used, in which the
positive electrode active material and lithium compound, as well as
the other optional components used as necessary, are mixed in a dry
state without using a solvent, and the obtained mixture is
subjected to press molding, after which a conductive adhesive is
used for attachment to the positive electrode power collector.
The coating solution of the positive electrode precursor may also
be prepared by dry blending all or a portion of each of the
starting materials containing the positive electrode active
material, and then adding water or an organic solvent, and/or a
liquid or slurry-like substance comprising a binder or dispersion
stabilizer dissolved or dispersed in them. The coating solution may
also be prepared by adding various starting powders containing the
positive electrode active material, to a liquid or slurry-like
substance comprising a binder or dispersion stabilizer dissolved or
dispersed in water or an organic solvent. The method of dry
blending may be, for example, premixing in which a ball mill or the
like is used to premix the positive electrode active material and
lithium compound, and a conductive filler if necessary, and the
low-conductivity lithium compound is coated with the conductive
material. This will help the lithium compound to decompose by the
positive electrode precursor in the lithium doping step described
below. When water is used as the solvent for the coating solution,
the coating solution may be rendered alkaline by addition of the
lithium compound, and therefore a pH regulator may be added as
necessary.
The method of dissolution or dispersion is not particularly
restricted, and a dispersing machine such as a homodisperser or
multiscrew disperser, planetary mixer, thin-film spinning
high-speed mixer or the like, may be suitably used. In order to
obtain a coating solution in a satisfactorily dispersed state, it
is preferred for the dispersion to be at a circumferential speed of
1 m/s to 50 m/s. It is preferred if the circumferential speed is 1
m/s or greater, because this will allow each material to
satisfactorily dissolve or disperse. It is also preferred if the
circumferential speed is no greater than 50 m/s, because each
material will be unlikely to be broken down by heat or shear force
during dispersion, and re-aggregation will be reduced.
The degree of dispersion of the coating solution is preferably to a
granularity of 0.1 .mu.m to 100 .mu.m, as measured with a fineness
gauge. The upper limit for the degree of dispersion is more
preferably to a granularity of no greater than 80 .mu.m, and more
preferably to a granularity of no greater than 50 .mu.m. A
granularity of 0.1 .mu.m or greater means that each of the material
powders containing positive electrode active materials are not
excessively crushed during preparation of the coating solution. In
addition, if the granularity is no greater than 100 .mu.m, there
will be less clogging during discharge of the coating solution and
less formation of streaks in the coating film, allowing more stable
coating.
The viscosity (.eta.b) of the coating solution of the positive
electrode precursor is preferably 1,000 mPas to 20,000 mPas, more
preferably 1,500 mPas to 10,000 mPas and even more preferably 1,700
mPas to 5,000 mPas. If the viscosity (.eta.b) of the coating
solution of the positive electrode precursor is 1,000 mPas or
higher, liquid dripping during formation of the coating film will
be suppressed, and the coating film width and thickness can be
satisfactorily controlled. If the viscosity (.eta.b) of the coating
solution of the positive electrode precursor is no higher than
20,000 mPas, there will be less pressure loss in the flow channel
of the coating solution when a coating machine is used, allowing
stable coating to be carried out, and allowing control to less than
the prescribed coating film thickness.
The TI value (thixotropy index value) of the coating solution of
the positive electrode precursor is preferably 1.1 or greater, more
preferably 1.2 or greater and even more preferably 1.5 or greater.
If the TI value of the coating solution of the positive electrode
precursor is 1.1 or greater, it will be possible to satisfactorily
control the coating film width and thickness.
The method of forming a coating film of the positive electrode
precursor is not particularly restricted, and a coating machine
such as a die coater, comma coater, knife coater or gravure coating
machine may be suitably used. The coating film may be formed by
monolayer coating or by multilayer coating. In the case of
multilayer coating, the coating solution compositions may be
adjusted so that the lithium compound content differs within each
layer of the coating film. The coating speed is preferably 0.1
m/min to 100 m/min, more preferably 0.5 m/min to 70 m/min and even
more preferably 1 m/min to 50 m/min. If the coating speed is 0.1
m/min or greater then stable coating will be possible, and if it is
no greater than 100 m/min then coating precision can be adequately
ensured.
The method of drying the coating film of the positive electrode
precursor is not particularly restricted, and a drying method such
as hot air drying or infrared ray (IR) drying may be suitably
employed. Drying of the coating film may be drying at a single
temperature, or it may be drying while varying the temperature in
different stages. Several drying methods may also be used in
combination for drying. The drying temperature is preferably
25.degree. C. to 200.degree. C., more preferably 40.degree. C. to
180.degree. C. and even more preferably 50.degree. C. to
160.degree. C. If the drying temperature is 25.degree. C. or
higher, it will be possible to adequately volatilize off the
solvent in the coating film. If the drying temperature is no higher
than 200.degree. C., it will be possible to reduce cracking of the
coating film by rapid volatilization of the solvent or
maldistribution of the binder by migration, and oxidation of the
positive electrode power collector or positive electrode active
material layer.
The method of pressing the positive electrode precursor is not
particularly restricted, and a pressing machine such as a hydraulic
press or vacuum pressing machine may be suitably used. The film
thickness, bulk density and electrode strength of the positive
electrode active material layer can be adjusted by the pressing
pressure, the gap, and the surface temperature of the pressed
portion, as described below.
The pressing pressure is preferably 0.5 kN/cm to 20 kN/cm, more
preferably 1 kN/cm to 10 kN/cm and even more preferably 2 kN/cm to
7 kN/cm. If the pressing pressure is 0.5 kN/cm or greater, it will
be possible to adequately increase the electrode strength. If the
pressing pressure is no greater than 20 kN/cm, distortion or
wrinkling will be unlikely to occur in the positive electrode
precursor, and adjustment of the positive electrode active material
layer to the desired film thickness and bulk density will be
easier.
The gap between the press rolls may be set to a desired value
depending on the film thickness of the dried positive electrode
precursor, so that the desired film thickness and bulk density of
the positive electrode active material layer is obtained.
The pressing speed may also be set to the desired speed, so as to
reduce distortion and wrinkling in the positive electrode
precursor. The surface temperature of the pressed portion may be
room temperature, or it may be heated instead, if necessary.
In the case of heating, the lower limit for the surface temperature
of the pressed portion is preferably at least the melting point of
the binder minus 60.degree. C., more preferably at least the
melting point of the binder minus 45.degree. C., and even more
preferably at least the melting point of the binder minus
30.degree. C. The upper limit for the surface temperature of the
pressed portion in the case of heating is also preferably no higher
than the melting point of the binder used plus 50.degree. C., more
preferably no higher than the melting point of the binder plus
30.degree. C., and even more preferably no higher than the melting
point of the binder plus 20.degree. C. For example, when PVdF
(polyvinylidene fluoride: melting point=150.degree. C.) is used as
the binder, heating of the surface of the pressed portion is to
preferably between 90.degree. C. and 200.degree. C., more
preferably between 105.degree. C. and 180.degree. C. and even more
preferably between 120.degree. C. and 170.degree. C. When a
styrene-butadiene copolymer (melting point=100.degree. C.) is used
as the binder, heating of the surface of the pressed portion is to
preferably between 40.degree. C. and 150.degree. C., more
preferably between 55.degree. C. and 130.degree. C. and even more
preferably between 70.degree. C. and 120.degree. C.
The melting point of the binder can be determined by the
endothermic peak position in DSC (Differential Scanning
Calorimetry). For example, using a "DSC7" differential scanning
calorimeter by Perkin-Elmer, 10 mg of sample resin is set in the
measuring cell and the temperature is increased from 30.degree. C.
to 250.degree. C. at a temperature-elevating rate of 10.degree.
C./min, in a nitrogen gas atmosphere, the melting point being the
endothermic peak temperature during the temperature elevation.
Pressing may also be carried out multiple times while varying the
conditions including the pressing pressure, gap, speed, and pressed
portion surface temperature.
The film thickness of the positive electrode active material layer
is preferably 20 .mu.m to 200 .mu.m for each side of the positive
electrode power collector, more preferably 25 .mu.m to 100 .mu.m
for each side, and even more preferably 30 .mu.m to 80 .mu.m. If
the film thickness of the positive electrode active material layer
is 20 .mu.m or greater, sufficient charge/discharge capacity can be
exhibited. If the film thickness of the positive electrode active
material layer is no greater than 200 .mu.m, low ion diffusion
resistance can be maintained in the electrode. Therefore, if the
film thickness of the positive electrode power collector layer is
20 .mu.m to 200 .mu.m, it will be possible to obtain an adequate
output characteristic, while also reducing the volume of the
nonaqueous lithium power storage element, thereby allowing the
energy density to be increased. The film thickness of the positive
electrode active material layer, when the positive electrode power
collector has through-holes or ruggedness, is the mean value of the
film thickness of the positive electrode active material layer for
each side at the sections of the positive electrode power collector
without through-holes or ruggedness.
[Positive Electrode]
The bulk density of the positive electrode active material layer at
the positive electrode after the lithium doping step described
below is in the range of preferably 0.50 g/cm.sup.3 or greater and
more preferably 0.55 g/cm.sup.3 to 1.3 g/cm.sup.3. If the bulk
density of the positive electrode active material layer is 0.50
g/cm.sup.3 or greater, it will be possible to exhibit high energy
density and to reduce the size of the nonaqueous lithium power
storage element. If the bulk density of the positive electrode
active material layer is no higher than 1.3 g/cm.sup.3, on the
other hand, diffusion of the nonaqueous electrolytic solution in
the pores in the positive electrode active material layer will be
adequate, and a high output characteristic will be obtained.
<Negative Electrode>
The negative electrode of this embodiment has a negative electrode
power collector and a negative electrode active material layer
containing a negative electrode active material, formed on one or
both sides thereof.
For the negative electrode of this embodiment, in the solid
.sup.7Li-NMR spectrum of the negative electrode active material
layer containing a graphite-based carbon material as the negative
electrode active material, in the spectral range of -10 ppm to 35
ppm, the maximum value of the peaks is between 4 ppm to 30 ppm, and
the amount of lithium per unit weight of the negative electrode
active material layer that has intercalated lithium ion (hereunder
also referred to as "amount of lithium in the negative electrode
active material layer"), as calculated by the peak area from 4 ppm
to 30 ppm, is preferably 0.10 mmol/g to 10.0 mmol/g, more
preferably 0.30 mmol/g to 9.0 mmol/g, even more preferably 0.50
mmol/g to 8.0 mmol/g, yet more preferably 0.80 mmol/g to 7.5 mmol/g
and most preferably 1.0 mmol/g to 7.0 mmol/g.
If the negative electrode of this embodiment contains a
graphite-based carbon material as the negative electrode active
material, and in the solid .sup.7Li-NMR spectrum of the negative
electrode active material layer, in the spectral range of -10 ppm
to 35 ppm, the maximum value of the peaks is between 4 ppm to 30
ppm, and also if the amount of lithium per unit weight of the
negative electrode active material layer that has intercalated
lithium ion, as calculated by the peak area from 4 ppm to 30 ppm,
is within the range specified above, then a nonaqueous lithium
power storage element using the negative electrode will exhibit a
high input/output characteristic and a high-load charge/discharge
cycle characteristic. While the principle is not completely
understood, and it is not our intention to be limited by theory,
the following is conjectured.
For solid .sup.7Li-NMR of the negative electrode active material
layer, the spectrum observed from 30 ppm to 60 ppm is due to
lithium ions intercalated within the carbon hexagonal mesh surface
of the graphite portion of the graphite-based carbon material.
Since lithium ion in the intercalated state strongly interacts with
the carbon hexagonal mesh surface, a large amount of energy is
necessary to release the lithium ion, and the resistance is
increased.
On the other hand, the spectrum observed from 4 ppm to 30 ppm in
solid .sup.7Li-NMR of the negative electrode active material layer
is thought to be derived from lithium ions intercalated in the
amorphous portion of the graphite-based carbon material, the
boundary between the graphite portion and the amorphous portion,
and within the carbon hexagonal mesh surface layer of the graphite
portion near the boundary, which are either mutually exchanged or
mutually interacting. Since such lithium ions in the intercalated
state weakly interact with carbon atoms, a large amount of energy
is not necessary to release the lithium ions. Moreover,
intercalation and release of lithium ions between the negative
electrode and the nonaqueous electrolytic solution is thought to
take place through the amorphous portion, which has more reaction
sites for the intercalated lithium ions than the graphite portion.
Therefore, it is believed, adjusting the maximum value of the peaks
in the spectral range of -10 ppm to 35 ppm in the solid
.sup.7Li-NMR spectrum of the negative electrode active material
layer to between 4 ppm to 30 ppm, and the amount of lithium, as
calculated by the peak area from 4 ppm to 30 ppm, to within the
range specified above, it is possible to reduce the input/output
resistance and exhibit a high input/output characteristic for a
nonaqueous lithium power storage element using the negative
electrode. In addition, lithium ion in the intercalated state can
adequately respond even during high-load charge/discharge cycling
in which high current charge/discharge is repeated, for the reason
described above, and a satisfactory high-load charge/discharge
cycle characteristic can be exhibited.
If the amount of lithium in the negative electrode active material
layer is 0.10 mmol/g or greater, for the reason explained above, a
nonaqueous lithium power storage element using a negative electrode
containing the negative electrode active material layer can exhibit
a high input/output characteristic and a high-load charge/discharge
cycle characteristic. On the other hand, if the amount of lithium
in the negative electrode active material layer is no greater than
10.0 mmol/g, then it will be possible to suppress self-discharged
release of lithium ions that have been intercalated in the negative
electrode active material. Thus, in the negative electrode of this
embodiment, it is possible to suppress reaction of lithium ions
released by self-discharge, with the nonaqueous electrolytic
solution in the negative electrode active material layer, and
increase in the coating film or deposit, thereby allowing a
nonaqueous lithium power storage element using the negative
electrode to exhibit a high high-load charge/discharge cycle
characteristic.
Throughout the present specification, the amount of lithium per
unit weight of the negative electrode active material layer that
has intercalated lithium ions, as obtained by the solid
.sup.7Li-NMR spectrum of the negative electrode active material
layer (the amount of lithium in the negative electrode active
material layer), can be calculated by the following method.
The measuring apparatus used for solid .sup.7Li-NMR may be a
commercially available apparatus. The spectrum is measured by the
single pulse method in a room temperature environment, with a
magic-angle spinning rotational speed of 14.5 kHz and an
irradiation pulse width set to a 45.degree. pulse. The repeated
latency during the measurement is set for adequate measurement.
A 1 mol/L aqueous lithium chloride solution is used as the shift
reference, and the shift position measured separately as an
external standard is defined as 0 ppm. During measurement of the 1
mol/L aqueous lithium chloride solution, the single pulse method is
used for spectral measurement with an irradiation pulse width set
to a 45.degree. pulse, without rotation of the sample.
The obtained solid .sup.7Li-NMR spectrum for the negative electrode
active material layer obtained by the method described above is
used to determine the peak areas for components in the range of 4
ppm to 30 ppm. The peak areas may then be divided by the peak area
for a 1 mol/L aqueous lithium chloride solution, with the same
sample height in the measuring rotor as during measurement of the
negative electrode active material layer, and further divided by
the weight of the negative electrode active material layer used for
measurement, to calculate the amount of lithium in the negative
electrode active material layer. Throughout the present
specification, the "weight of the negative electrode active
material layer" is the weight of the negative electrode active
material layer including lithium ion intercalated in the negative
electrode active material layer and/or the coating film or
accumulated deposit on the negative electrode active material
layer.
For this embodiment, the mean distance between the centers of
gravity of the voids (hereunder also, "r.sub.p") obtained by SEM of
the cross-section of the negative electrode active material layer,
is preferably 1 .mu.m to 10 .mu.m, more preferably between 1.3
.mu.m and 8 .mu.m, inclusive, even more preferably between 1.5
.mu.m and 6 .mu.m, inclusive, yet more preferably between 1.7 .mu.m
and 5 .mu.m, inclusive and most preferably between 1.9 .mu.m and 4
.mu.m, inclusive.
The nonaqueous lithium power storage element of this embodiment,
using a positive electrode containing a lithium compound other than
the positive electrode active material, and a negative electrode
having the mean distance between the centers of gravity of the
voids, obtained by SEM of a cross-section of the negative electrode
active material layer, adjusted to within a specified range,
exhibits a high input/output characteristic and a high-load
charge/discharge cycle characteristic. While the principle is not
completely understood, and it is not our intention to be limited by
theory, the following is conjectured. It is believed that the mean
distance between the centers of gravity of the voids, obtained by
SEM of a cross-section of the negative electrode active material
layer, represents the distribution of the nonaqueous electrolytic
solution held in the negative electrode active material layer.
Consequently, by adjusting the mean distance between the centers of
gravity of the voids obtained by SEM of a cross-section of the
negative electrode active material layer to be within an
appropriate range, it is possible hold a suitable amount of the
nonaqueous electrolytic solution around the negative electrode
active material. It is therefore possible to avoid lithium ion
deficiency surrounding the negative electrode active material even
during high current charge/discharge or high-load charge/discharge
cycling, and to exhibit a high input/output characteristic and a
high-load charge/discharge cycle characteristic. By using a
negative electrode wherein the mean distance between the centers of
gravity of the voids, obtained by SEM of a cross-section of the
negative electrode active material layer, is adjusted to 1 .mu.m or
greater, active products such as fluorine ions (HF, for example)
generated at the positive electrode during high-load
charge/discharge cycling will easily diffuse in the negative
electrode active material layer. Therefore, the active products
such as fluorine ions react with the lithium ions intercalated in
the negative electrode active material and with the nonaqueous
electrolytic solution, in the negative electrode active material
layer, thus increasing the coating film and accumulated deposit
resulting from reductive decomposition of the nonaqueous
electrolytic solution. This results in deterioration of the
high-load charge/discharge cycle characteristic. However, by adding
a lithium compound other than the positive electrode active
material to the positive electrode, the lithium compound traps such
active products such as fluorine ions, allowing increase in the
coating film and accumulated deposit in the negative electrode
active material layer to be suppressed, and allowing a satisfactory
high-load charge/discharge cycle characteristic to be
exhibited.
If the mean distance between the centers of gravity of the voids
obtained by SEM of a cross-section of the negative electrode active
material layer is 1 .mu.m or greater, the sizes of the voids will
increase and a sufficient amount of nonaqueous electrolytic
solution will be able to be retained in the voids, thereby allowing
a high input/output characteristic and a high-load charge/discharge
cycle characteristic to be exhibited, for the reason explained
above. If the mean distance between the centers of gravity of the
voids obtained by SEM of a cross-section of the negative electrode
active material layer is no greater than 10 .mu.m, a suitable
amount of nonaqueous electrolytic solution will be dispersed in the
negative electrode active material layer, thereby allowing a high
input/output characteristic and a high-load charge/discharge cycle
characteristic to be exhibited, for the reason explained above.
Throughout the present specification, the mean distance between the
centers of gravity of the voids obtained by SEM of a cross-section
of the negative electrode active material layer can be calculated
by the following method.
The sample used for measurement may be the negative electrode
before it is incorporated into the nonaqueous lithium power storage
element (hereunder also referred to as "negative electrode before
use"), or it may be the negative electrode incorporated in the
nonaqueous lithium power storage element (hereunder also referred
to as "negative electrode after use").
When the negative electrode incorporated in the nonaqueous lithium
power storage element is to be used as the measuring sample, the
following method, for example, is preferably used as pretreatment
of the measuring sample.
First, the nonaqueous lithium power storage element is disassembled
under an inert atmosphere such as argon, and the negative electrode
is removed. The removed negative electrode is immersed in a linear
carbonate (such as methyl ethyl carbonate or dimethyl carbonate),
the nonaqueous electrolytic solution and lithium salt are removed
and air-drying is carried out. Next, the following method (1), (2)
or (3) is preferably used.
(1) The obtained negative electrode is immersed in a mixed solvent
composed of methanol and isopropanol to inactivate the lithium ion
intercalated in the negative electrode active material, and
air-drying is carried out. Next, using vacuum drying or the like,
the linear carbonate and organic solvent in the obtained negative
electrode are removed to obtain a measuring sample.
(2) Using the obtained negative electrode as the working electrode
and metal lithium as the counter electrode and reference electrode,
they are immersed in the nonaqueous electrolytic solution under an
inert atmosphere such as argon, to fabricate an electrochemical
cell. A charger-discharger is used for adjustment of the obtained
electrochemical cell, so that the negative electrode potential (vs.
Li/Li.sup.+) is in the range of 1.5 V to 3.5 V. Next, the negative
electrode is removed from the electrochemical cell under an inert
atmosphere such as argon and immersed in a linear carbonate to
remove the nonaqueous electrolytic solution and lithium salt, and
air-drying is carried out. Next, vacuum drying or the like is used
to remove the linear carbonate in the obtained negative electrode,
to obtain a measuring sample.
(3) The obtained negative electrode may be used directly as the
measuring sample. In this case, the formation of the cross-section
of the negative electrode active material layer and the SEM
observation described below are preferably carried out under an
inert atmosphere such as argon.
Next, when a horizontal plane with respect to the direction of
lamination of the negative electrode power collector and negative
electrode active material layer is to be the cross section, and a
plane crossing perpendicular to the horizontal plane is to be the
flat section, as shown in FIG. 1, the measuring sample obtained as
explained above is used to form a cross-section of the negative
electrode active material layer. The method of forming the
cross-section of the negative electrode active material layer is
not particularly restricted so long as it is a method that can
minimize damage to the cross-section of the negative electrode
active material layer by formation or processing of the
cross-section, but it is preferred to use a processing method using
an ion beam (for example, BIB (Broad Ion Beam) processing or FIB
(Focused Ion Beam) processing), or to use a precision machining
polisher, ultramicrotome or the like. From the viewpoint of
minimizing damage by formation and processing of the cross-section
of the negative electrode active material layer it is particularly
preferred to use BIB processing with an argon ion beam. A method of
forming a cross-section of a negative electrode active material
layer using BIB processing is as follows. An argon ion beam is
irradiated from above the flat section of the negative electrode,
and a cross-section of the negative electrode active material layer
perpendicular to the flat section of the negative electrode is
created along the edge of a masking shield (mask) set directly
above the flat section of the negative electrode.
The formed negative electrode active material layer cross-section
is observed with a scanning electron microscope (SEM) to obtain an
SEM image of the cross-section of the negative electrode active
material layer. If necessary, a lower detector capable of lowering
the detection sensitivity for the internal structure of the
negative electrode active material layer that is observed between
the negative electrode active material may be used, from the
viewpoint of facilitating image analysis such as binarization,
described below.
The obtained cross-section of the negative electrode active
material layer SEM image is then subjected to image analysis. The
image analysis tool is not particularly restricted so long as it
can carry out the processing described below, and an IP-1000 by
Asahi Kasei Corp. (software: A-Zou Kun), or ImageJ, may be
used.
A region for image analysis is extracted from a cross-section of
the negative electrode active material layer in an SEM image at an
observational magnification of 1,000.times. to 10,000x, and
preferably 3,000x. If necessary, before performing the binarization
described below, a median filter or the like may be used for the
extracted region to remove trace noise included in the image. A
median filter, for the purpose of the present specification, is the
procedure of substituting the luminance of a pixel of interest with
the median luminance of the peripheral 9 pixels (3 pixels.times.3
pixels).
Next, the extracted region is subjected to binarization processing,
in which an image with contrast is converted to two-tone (for
example, black and white). Binarization is performed by adjusting
the contrast so that the minimum and maximum values in a luminance
histogram of the extracted region are included, and sections of the
extracted region corresponding to voids are dark while the sections
corresponding to the negative electrode active material are light.
In the binarization, with the color tone positioned at the bottom
of the valley of the luminance histogram in the extracted region as
the cutoff value, gradation 1 (for example, white) is assigned if
the luminance of each pixel is above the cutoff value, while
gradation 2 is assigned if it is below (for example, black). In
this case, gradation 2 (for example, black) corresponds to a
void.
The sections with gradation 2 in the binarized image are treated as
voids, and the mean distance between the centers of gravity of the
voids is calculated by the following method. Voids having larger
areas than 0.2 .mu.m.sup.2 are used in order to eliminate the
effect of fine voids arising from the conductive filler, for
example, and the centers of gravity of adjacent voids are connected
with straight lines, calculating the mean value of the lengths of
the line segments (distances between centers of gravity) as the
mean distance between the centers of gravity of the voids. The
method of connecting the centers of gravity is not randomly, but in
a pattern known as a Delaunay diagram or Delaunay triangulation.
Connecting the centers of gravity forms polygons, which are
triangular except in special cases.
[Negative Electrode Active Material Layer]
The negative electrode active material layer contains the negative
electrode active material, but it may also contain optional
components such as a conductive filler, binder and dispersion
stabilizer, as necessary.
(Negative Electrode Active Material)
The negative electrode active material used may be a substance
capable of intercalating and releasing lithium ions. Negative
electrode active materials include, specifically, carbon materials,
titanates, silicon, silicon oxides, silicon alloys, silicon
compounds, tin and tin compounds. The content of the carbon
material with respect to the total weight of the negative electrode
active material is preferably 50 weight % or greater and more
preferably 70 weight % or greater. The carbon material content may
be 100 weight %, but from the viewpoint of obtaining a satisfactory
effect by combined use with other materials, it is preferably, for
example, no greater than 90 weight %, and may even be 80 weight %
or lower.
The negative electrode active material is preferably doped with
lithium ion. The lithium ion doped in the negative electrode active
material, for this embodiment, includes three major forms.
The first form is lithium ion that is intercalated in advance in
the negative electrode active material, as a design value, before
fabrication of the nonaqueous lithium power storage element.
The second form is lithium ion intercalated in the negative
electrode active material during fabrication and shipping of the
nonaqueous lithium power storage element.
The third form is lithium ion intercalated in the negative
electrode active material after the nonaqueous lithium power
storage element has been used as a device.
By doping the negative electrode active material with lithium ion
it is possible to satisfactorily control the capacitance and
operating voltage of the obtained nonaqueous lithium power storage
element.
Examples of carbon materials include non-graphitizable carbon
materials (hard carbon); easily graphitizable carbon materials
(soft carbon); carbon black; carbon nanoparticles; activated
carbon; graphite-based carbon materials; amorphous carbonaceous
materials such as polyacene-based materials; carbonaceous materials
obtained by heat treatment of carbonaceous material precursors such
as petroleum-based pitch, coal-based pitch, mesocarbon microbeads,
coke and synthetic resins (for example, phenol resins); thermal
decomposition products of furfuryl alcohol resins or novolac
resins; fullerenes; carbon nanohorns; and carbon materials that are
composites of the foregoing.
Examples of graphite-based carbon materials include graphite
materials such as artificial graphite, natural graphite, low
crystal graphite, graphitized mesophase carbon microspheres,
graphite whiskers and high specific surface area graphite, as well
as carbon materials obtained by subjecting these graphite materials
to the amorphous portion-forming method described below.
The method of forming the amorphous portion of the graphite-based
carbon material is not particularly restricted, and may be a method
of compositing the graphite material and the carbonaceous material
described below; a method of carrying out physical surface
modification of the graphite material by laser, plasma, corona
treatment or the like; a method of immersing the graphite material
in an acid or alkali solution and heating for chemical surface
modification of the graphite material; or a method of forming
graphite and amorphous materials in a random (vitreous) fashion by
a calcination pattern during graphitization of the starting
material for the graphite-based carbon material, such as needle
coke (for example, rapid temperature increase in the range of
2,000.degree. C. to 3,000.degree. C., followed by rapid temperature
lowering to 100.degree. C. or below). The amorphous portion may be
formed on the surface of the graphite-based carbon material or it
may be formed inside the graphite-based carbon material, but
preferably it is formed on the surface of the graphite-based carbon
material for the reason explained above.
Preferred among these, from the viewpoint of lowering the
resistance of the negative electrode, is a composite carbon
material which is obtained by heat treating one or more of the
aforementioned carbon materials (hereunder referred to as "base
material") in the copresence of the carbonaceous material
precursor, to form a composite of the base material with the
carbonaceous material derived from the carbonaceous material
precursor. The carbonaceous material precursor is not particularly
restricted so long as it is converted to a carbonaceous material by
heat treatment, but petroleum-based pitch or coal-based pitch is
especially preferred. Before the heat treatment, the base material
and the carbonaceous material precursor may be mixed at a
temperature higher than the melting point of the carbonaceous
material precursor. The heat treatment temperature may be any
temperature such that the components generated when the
carbonaceous material precursor that is used volatilizes or
thermally decomposes, form a carbonaceous material, and it is
preferably 400.degree. C. to 2,500.degree. C., more preferably
500.degree. C. to 2,000.degree. C., and even more preferably
550.degree. C. to 1,500.degree. C. The atmosphere for heat
treatment is not particularly restricted, but is preferably a
non-oxidizing atmosphere.
Preferred examples for the composite carbon material are composite
carbon materials 1 and 2 described below. Either of these may be
selected for use, or both may be used in combination.
(Composite Carbon Material 1)
In the present specification, composite carbon material 1 is a
composite carbon material using at least one type of carbon
material with a BET specific surface area of 100 m.sup.2/g to 3,000
m.sup.2/g as the base material. The base material of composite
carbon material 1 is not particularly restricted so long as it has
a BET specific surface area of 100 m.sup.2/g to 3,000 m.sup.2/g,
and activated carbon, carbon black, molded porous carbon, high
specific surface area graphite or carbon nanoparticles may be
suitably used.
The BET specific surface area of the composite carbon material 1 is
preferably 100 m.sup.2/g to 1,500 m.sup.2/g, more preferably 150
m.sup.2/g to 1,100 m.sup.2/g, and even more preferably 180
m.sup.2/g to 550 m.sup.2/g. If the BET specific surface area of the
composite carbon material 1 is 100 m.sup.2/g or greater, suitable
pores will be maintained and diffusion of lithium ions in the
nonaqueous electrolytic solution will be satisfactory, and
therefore a high input/output characteristic can be exhibited and
reaction sites between lithium ions in the nonaqueous electrolytic
solution can be adequately increased, thereby allowing a high
input/output characteristic to be exhibited. If the BET specific
surface area of the composite carbon material 1 is no greater than
1,500 m.sup.2/g, the lithium ion charge/discharge efficiency will
be increased and excessive reductive decomposition of the
nonaqueous electrolytic solution can be suppressed, so that
impairment of the high-load charge/discharge cycle characteristic
can be minimized.
The weight ratio of the carbonaceous material with respect to the
base material in composite carbon material 1 is preferably 10
weight % to 200 weight %, more preferably 12 weight % to 180 weight
%, even more preferably 15 weight % to 160 weight % and most
preferably 18 weight % to 150 weight %. If the weight ratio of the
carbonaceous material is 10 weight % or greater, it will be
possible to suitably fill the micropores of the base material with
the carbonaceous material, and the lithium ion charge/discharge
efficiency will be increased, therefore allowing a high-load
charge/discharge cycle characteristic to be exhibited. If the
weight ratio of the carbonaceous material with respect to the base
material is no greater than 200 weight %, it will be possible to
suitably maintain the pores and the lithium ion diffusion will be
satisfactory, and therefore a high input/output characteristic can
be exhibited.
The lithium ion doping amount per unit weight of composite carbon
material 1 is preferably 530 mAh/g to 2,500 mAh/g, more preferably
620 mAh/g to 2,100 mAh/g, even more preferably 760 mAh/g to 1,700
mAh/g and yet more preferably 840 mAh/g to 1,500 mAh/g.
Doping lithium ion in the negative electrode will lower the
potential of the negative electrode. Thus, when a negative
electrode containing composite carbon material 1 doped with lithium
ion is combined with a positive electrode, the voltage of the
nonaqueous lithium power storage element is increased and the
utilizable capacity of the positive electrode is increased.
Therefore, the capacitance and energy density of the obtained
nonaqueous lithium power storage element increases.
If the lithium ion doping amount per unit weight of the composite
carbon material 1 is 530 mAh/g or greater, lithium ion in the
composite carbon material 1 will be satisfactorily doped even at
irreversible sites where lithium ion cannot be desorbed after once
being inserted, and it will also be possible to reduce the amount
of composite carbon material 1 per amount of lithium. The film
thickness of the negative electrode can therefore be reduced and
high energy density can be obtained. As the doping amount
increases, the negative electrode potential decreases and the
input/output characteristic, energy density and durability
increase.
If the lithium ion doping amount per unit weight of the composite
carbon material 1 is no greater than 2,500 mAh/g, side-effects of
lithium metal deposition and the like will be less likely to
occur.
Composite carbon material 1a using activated carbon as the base
material will now be described as a preferred example of composite
carbon material 1.
Composite carbon material 1a preferably satisfies
0.010.ltoreq.V.sub.m1.ltoreq.0.300,
0.001.ltoreq.V.sub.m2.ltoreq.0.650, where V.sub.m1 (cc/g) is the
mesopore volume due to pores with diameters of 20 .ANG. to 500
.ANG., as calculated by the BJH method, and V.sub.m2 (cc/g) is the
micropore volume due to pores with diameters of smaller than 20
.ANG. as calculated by the MP method.
The mesopore volume V.sub.m1 more preferably satisfies
0.010.ltoreq.V.sub.m1.ltoreq.0.225 and even more preferably
0.010.ltoreq.V.sub.m1.ltoreq.0.200. The micropore volume V.sub.m2
more preferably satisfies 0.001.ltoreq.V.sub.m2.ltoreq.0.200, even
more preferably 0.001.ltoreq.V.sub.m2.ltoreq.0.150 and most
preferably 0.001.ltoreq.V.sub.m2.ltoreq.0.100.
If the mesopore volume V.sub.m1 is no greater than 0.300 cc/g it
will be possible to increase the BET specific surface area and
increase the lithium ion doping amount, while also increasing the
bulk density of the negative electrode. As a result, the negative
electrode can be made into a thin-film. If the micropore volume
V.sub.m2 is no greater than 0.650 cc/g, it will be possible to
maintain high charge/discharge efficiency for lithium ions. On the
other hand, if the mesopore volume V.sub.m1 and micropore volume
V.sub.m2 satisfy 0.010.ltoreq.V.sub.m1 and 0.001.ltoreq.V.sub.m2,
then a high input/output characteristic can be obtained.
The BET specific surface area of composite carbon material 1a is
preferably 100 m.sup.2/g to 1,500 m.sup.2/g, more preferably 150
m.sup.2/g to 1,100 m.sup.2/g, and even more preferably 180
m.sup.2/g to 550 m.sup.2/g. If the BET specific surface area of the
composite carbon material 1a is 100 m.sup.2/g or greater, suitable
pores will be maintained and diffusion of lithium ions in the
nonaqueous electrolytic solution will be satisfactory, and
therefore a high input/output characteristic can be exhibited and
reaction sites between lithium ions in the nonaqueous electrolytic
solution can be adequately increased, thereby allowing a high
input/output characteristic to be exhibited. If the BET specific
surface area of the composite carbon material 1a is no greater than
1,500 m.sup.2/g, the lithium ion charge/discharge efficiency will
be increased and excessive reductive decomposition of the
nonaqueous electrolytic solution can be suppressed, so that
impairment of the high-load charge/discharge cycle characteristic
can be minimized.
The mean pore size of composite carbon material 1a is preferably 20
.ANG. or larger, more preferably 25 .ANG. or larger and even more
preferably 30 .ANG. or larger, from the viewpoint of obtaining a
high input/output characteristic. The mean pore size of composite
carbon material 1a is preferably no larger than 65 .ANG. and more
preferably no larger than 60 .ANG., from the viewpoint of obtaining
high energy density.
The mean particle diameter of composite carbon material 1a is
preferably 1 .mu.m to 10 .mu.m, the lower limit being more
preferably 2 .mu.m or larger and even more preferably 2.5 .mu.m or
larger, and the upper limit being more preferably no larger than 6
.mu.m and even more preferably no larger than 4 .mu.m. If the mean
particle diameter of composite carbon material 1a is 1 .mu.m to 10
.mu.m, then satisfactory durability will be maintained.
For composite carbon material 1a, the atomic ratio of
hydrogen/carbon atom (H/C) is preferably 0.05 to 0.35 and more
preferably 0.05 to 0.15. If H/C for composite carbon material 1a is
0.35 or smaller, the structure of the carbonaceous material
adhering to the activated carbon surface, which is typically a
polycyclic aromatic conjugated structure, will satisfactorily
develop and the capacitance (energy density) and charge/discharge
efficiency will increase. If H/C for composite carbon material 1a
is 0.05 or larger, there will be no excessive carbonization, and
therefore satisfactory energy density will be obtained. The H/C
ratio is measured with an elemental analyzer.
Composite carbon material 1a has an amorphous structure derived
from the activated carbon of the base material, but it
simultaneously also has a crystal structure derived mainly from the
coated carbonaceous material. Based on wide-angle X-ray
diffraction, in the composite carbon material 1a, preferably the
plane spacing d.sub.002 of the (002) plane is 3.60 .ANG. to 4.00
.ANG., and the crystallite size Lc in the c-axis direction obtained
from the half width of the peak is 8.0 .ANG. to 20.0 .ANG.; and
more preferably d.sub.002 is 3.60 .ANG. to 3.75 .ANG., and the
crystallite size Lc in the c-axis direction obtained from the half
width of the peak is 11.0 .ANG. to 16.0 .ANG..
The activated carbon used as the base material for composite carbon
material 1a is not particularly restricted so long as the obtained
composite carbon material 1a exhibits the desired properties. For
example, it is possible to use a commercially available product
obtained from a petroleum-based, coal-based, plant-based or
polymer-based starting material as the activated carbon of
composite carbon material 1a. It is particularly preferred to use
activated carbon powder having a mean particle diameter of 1 .mu.m
to 15 .mu.m. The mean particle diameter of the activated carbon
powder is more preferably 2 .mu.m to 10 .mu.m.
In order to obtain composite carbon material 1a having the pore
distribution range specified for this embodiment, the pore
distribution of the activated carbon used as the base material is
important.
The activated carbon used as the base material for composite carbon
material 1a preferably satisfies 0.050.ltoreq.V.sub.1.ltoreq.0.500,
0.005.ltoreq.V.sub.2.ltoreq.1.000 and
0.2.ltoreq.V.sub.1/V.sub.2.ltoreq.20.0, where V.sub.1 (cc/g) is the
mesopore volume due to pores with diameters of 20 .ANG. to 500
.ANG., as calculated by the BJH method, and V.sub.2 (cc/g) is the
micropore volume due to pores with diameters of smaller than 20
.ANG. as calculated by the MP method.
The mesopore volume V.sub.1 more preferably satisfies
0.050.ltoreq.V.sub.1.ltoreq.0.350 and more preferably
0.100.ltoreq.V.sub.1.ltoreq.0.300. The micropore volume V.sub.2
more preferably satisfies 0.005.ltoreq.V.sub.2.ltoreq.0.850 and
more preferably 0.100.ltoreq.V.sub.2.ltoreq.0.800. The mesopore
volume/micropore volume ratio satisfies preferably
0.22.ltoreq.V.sub.1/V.sub.2.ltoreq.15.0 and more preferably
0.25.ltoreq.V.sub.1/V.sub.2.ltoreq.10.0. When the mesopore volume
V.sub.1 of the activated carbon is 0.500 or smaller and the
micropore volume V.sub.2 is 1.000 or smaller, coating a suitable
amount of carbonaceous material will be adequate for obtaining a
pore structure for the composite carbon material 1a according to
this embodiment, and it will therefore tend to be easier to control
the pore structure. When the mesopore volume V.sub.1 of the
activated carbon is 0.050 or greater and the micropore volume
V.sub.2 is 0.005 or greater, the desired pore structure can be
easily obtained if V.sub.1/V.sub.2 is 0.2 or greater and
V.sub.1/V.sub.2 is no greater than 20.0.
A carbonaceous material precursor to be used as a starting material
for composite carbon material 1a is a solid, liquid or
solvent-soluble organic material that can be coated as a
carbonaceous material onto activated carbon by heat treatment. The
carbonaceous material precursor may be, for example, pitch,
mesocarbon microbeads, coke or a synthetic resin such as a phenol
resin, for example. Among such carbonaceous material precursors,
the use of inexpensive pitch is preferred in terms of production
cost. Pitch is largely classified as petroleum-based pitch or
coal-based pitch. Examples of petroleum-based pitch include crude
oil distillation residue, fluid catalytic cracking residue (decant
oil and the like), bottom oil from thermal crackers, and ethylene
tar obtained during naphtha cracking.
When pitch is used, composite carbon material 1a can be obtained by
heat treatment of the pitch in the co-presence of activated carbon,
causing thermal reaction of the volatile components and thermal
decomposition components of the pitch on the surface of the
activated carbon to coat the carbonaceous material onto the
activated carbon. In this case, coating of the volatile components
or thermal decomposition components of the pitch inside the pores
of the activated carbon proceeds at a temperature of about 200 to
500.degree. C., and the coated components undergo reaction to form
a carbonaceous material at about 400.degree. C. or higher. The peak
temperature during heat treatment (maximum ultimate temperature)
may be appropriately set depending on the properties of the
composite carbon material 1a to be obtained, the thermal reaction
pattern and the thermal reaction atmosphere, but it is preferably
400.degree. C. or higher, more preferably 450.degree. C. to
1,000.degree. C. and even more preferably about 500 to 800.degree.
C. The time for which the peak temperature is maintained during
heat treatment is preferably 30 minutes to 10 hours, more
preferably 1 hour to 7 hours and even more preferably 2 hours to 5
hours. For example, with heat treatment at a peak temperature of
about 500 to 800.degree. C. over a period of 2 hours to 5 hours,
the carbonaceous material that has been coated onto the activated
carbon surface is potentially converted to polycyclic aromatic
hydrocarbons.
The softening point of the pitch is preferably between 30.degree.
C. and 250.degree. C., and more preferably between 60.degree. C.
and 130.degree. C. Pitch with a softening point of 30.degree. C. or
higher will allow precise charging to be carried out without
impairing the handleability. Pitch with a softening point of no
higher than 250.degree. C. comprises a relatively large number of
low molecular compounds, and therefore using pitch with a softening
point of no higher than 250.degree. C. will allow coating even to
the relatively fine pores in the activated carbon.
The specific method for producing composite carbon material 1a may
be, for example, a method in which activated carbon is heat treated
in an inert atmosphere containing a hydrocarbon gas volatilized
from the carbonaceous material precursor, and coated with the
carbonaceous material in a gas phase. It may instead be a method in
which the activated carbon and carbonaceous material precursor are
pre-mixed and then heat treated, or the carbonaceous material
precursor dissolved in a solvent is coated onto the activated
carbon and dried, and then heat treated.
The weight ratio of the carbonaceous material with respect to the
activated carbon in composite carbon material 1a is preferably 10
weight % to 100 weight % and more preferably 15 weight % to 80
weight %. If the weight ratio of the carbonaceous material is 10
weight % or greater, it will be possible to suitably fill the
micropores of the activated carbon with the carbonaceous material,
and the charge/discharge efficiency of lithium ions will be
increased, thus resulting in less impairment of the high-load
charge/discharge cycle characteristic. If the weight ratio of the
carbonaceous material is no greater than 100 weight %, the pores in
the composite carbon material 1a will be suitably conserved and a
high specific surface area will be maintained. The lithium ion
doping amount can therefore be increased, allowing high output
density and high durability to be maintained even if the negative
electrode is a thin-film.
(Composite Carbon Material 2)
In the present specification, composite carbon material 2 is a
composite carbon material using at least one type of carbon
material with a BET specific surface area of 0.5 m.sup.2/g to 80
m.sup.2/g as the base material. The base material of composite
carbon material 2 is not particularly restricted so long as it has
a BET specific surface area of 0.5 m.sup.2/g to 80 m.sup.2/g, and
graphite materials, hard carbon, soft carbon, carbon black or the
like may be suitably used.
The BET specific surface area of composite carbon material 2 is
preferably 1 m.sup.2/g to 50 m.sup.2/g, more preferably 1.5
m.sup.2/g to 40 m.sup.2/g and even more preferably 2 m.sup.2/g to
25 m.sup.2/g. If the BET specific surface area of composite carbon
material 2 is 1 m.sup.2/g or greater, it will be possible to ensure
an adequately large number of reaction sites with lithium ions in
the nonaqueous electrolytic solution, thereby allowing a high
input/output characteristic to be exhibited. If the BET specific
surface area of composite carbon material 2 is no greater than 50
m.sup.2/g, the lithium ion charge/discharge efficiency will be
increased and decomposition reaction of the nonaqueous electrolytic
solution during charge/discharge will be inhibited, thus allowing a
high high-load charge/discharge cycle characteristic to be
exhibited.
The mean particle diameter of composite carbon material 2 is
preferably 1 .mu.m to 10 .mu.m, more preferably 2 .mu.m to 8 .mu.m
and even more preferably 3 .mu.m to 6 .mu.m. If the mean particle
diameter of composite carbon material 2 is 1 .mu.m or larger it
will be possible to increase the lithium ion charge/discharge
efficiency, and to thus exhibit a high high-load charge/discharge
cycle characteristic. If the mean particle diameter of composite
carbon material 2 is no larger than 10 .mu.m, the number of
reaction sites between composite carbon material 2 and lithium ions
in the nonaqueous electrolytic solution will increase, allowing a
high input/output characteristic to be exhibited.
The weight ratio of the carbonaceous material with respect to the
base material in composite carbon material 2 is preferably 1 weight
% to 30 weight %, more preferably 1.2 weight % to 25 weight % and
even more preferably 1.5 weight % to 20 weight %. If the weight
ratio of the carbonaceous material is 1 weight % or greater, the
number of reaction sites with lithium ion in the nonaqueous
electrolytic solution can be adequately increased by the
carbonaceous material, and desolvation of the lithium ion will be
facilitated, thus allowing a high input/output characteristic to be
exhibited. If the weight ratio of the carbonaceous material is no
greater than 20 weight %, on the other hand, it will be possible to
satisfactorily maintain solid diffusion of lithium ions between the
carbonaceous material and base material, and therefore a high
input/output characteristic can be exhibited. In addition, the
lithium ion charge/discharge efficiency can be increased, and
consequently a high high-load charge/discharge cycle characteristic
can be exhibited.
The lithium ion doping amount per unit weight of composite carbon
material 2 is preferably 50 mAh/g to 700 mAh/g, more preferably 70
mAh/g to 650 mAh/g, even more preferably 90 mAh/g to 600 mAh/g and
yet more preferably 100 mAh/g to 550 mAh/g.
Doping lithium ion in the negative electrode will lower the
potential of the negative electrode. Thus, when a negative
electrode containing composite carbon material 2 doped with lithium
ion is combined with a positive electrode, the voltage of the
nonaqueous lithium power storage element is increased and the
utilizable capacity of the positive electrode is increased.
Therefore, the capacitance and energy density of the obtained
nonaqueous lithium power storage element increases.
If the lithium ion doping amount per unit weight of composite
carbon material 2 is 50 mAh/g or greater, lithium ion will be
satisfactorily doped even at irreversible sites where lithium ion
in the composite carbon material 2 cannot be desorbed after once
being inserted, and therefore high energy density can be obtained.
As the doping amount increases, the negative electrode potential
decreases and the input/output characteristic, energy density and
durability increase.
If the lithium ion doping amount per unit weight of the composite
carbon material 2 is no greater than 700 mAh/g, side-effects of
lithium metal deposition and the like will be less likely to
occur.
Composite carbon material 2a using a graphite material as the base
material will now be explained as a preferred example of composite
carbon material 2a.
The BET specific surface area of composite carbon material 2a is
preferably 1 m.sup.2/g to 50 m.sup.2/g, more preferably 1 m.sup.2/g
to 20 m.sup.2/g and even more preferably 1 m.sup.2/g to 15
m.sup.2/g. If the BET specific surface area of composite carbon
material 2a is 1 m.sup.2/g or greater, it will be possible to
ensure an adequately large number of reaction sites with lithium
ions in the nonaqueous electrolytic solution, thereby allowing a
high input/output characteristic to be exhibited. If the BET
specific surface area of composite carbon material 2a is no greater
than 50 m.sup.2/g, the lithium ion charge/discharge efficiency will
be increased and decomposition reaction of the nonaqueous
electrolytic solution during charge/discharge will be inhibited,
thus allowing a high high-load charge/discharge cycle
characteristic to be exhibited.
The mean pore size of composite carbon material 2a is preferably
1.5 nm to 25 nm, more preferably 2 nm to 22 nm, even more
preferably 3 nm to 20 nm and most preferably 3.5 nm to 18 nm. If
the mean pore size of composite carbon material 2a is 1.5 nm or
greater, there will be more pores with sizes larger than lithium
ion solvated in the nonaqueous electrolytic solution (approximately
0.9 nm to 1.2 nm), and therefore diffusion of the solvated lithium
ion in the composite carbon material 2a will be satisfactory, and a
nonaqueous lithium power storage element using it can exhibit a
high input/output characteristic. On the other hand, if the mean
pore size of the composite carbon material is no larger than 25 nm,
the bulk density of the negative electrode active material layer
using it can be sufficiently increased, and therefore high energy
density can be exhibited.
The mean particle diameter of composite carbon material 2a is
preferably 1 .mu.m to 10 .mu.m, more preferably 2 .mu.m to 8 .mu.m
and even more preferably 3 .mu.m to 6 .mu.m. If the mean particle
diameter of composite carbon material 2a is 1 .mu.m or larger it
will be possible to increase the lithium ion charge/discharge
efficiency, and to thus exhibit a high high-load charge/discharge
cycle characteristic. If the mean particle diameter of composite
carbon material 2a is no larger than 10 .mu.m, the number of
reaction sites with lithium ions in the nonaqueous electrolytic
solution will increase, allowing a high input/output characteristic
to be exhibited.
The weight ratio of the carbonaceous material with respect to the
graphite material in composite carbon material 2a is preferably 1
weight % to 20 weight %, more preferably 1.2 weight % to 15 weight
%, even more preferably 1.5 weight % to 10 weight % and most
preferably 2 weight % to 5 weight %. If the weight ratio of the
carbonaceous material is 1 weight % or greater, the number of
reaction sites with lithium ion in the nonaqueous electrolytic
solution can be adequately increased by the carbonaceous material,
and desolvation of the lithium ion will be facilitated, thus
allowing a high input/output characteristic to be exhibited. If the
weight ratio of the carbonaceous material is no greater than 20
weight %, it will be possible to satisfactorily maintain solid
diffusion of lithium ions between the carbonaceous material and
graphite material, and therefore a high input/output characteristic
can be exhibited. In addition, the lithium ion charge/discharge
efficiency can be increased, and consequently a high high-load
charge/discharge cycle characteristic can be exhibited.
The lithium ion doping amount per unit weight of composite carbon
material 2a is preferably 50 mAh/g to 700 mAh/g, more preferably 70
mAh/g to 650 mAh/g, even more preferably 90 mAh/g to 600 mAh/g and
yet more preferably 100 mAh/g to 550 mAh/g.
Doping lithium ion in the negative electrode will lower the
potential of the negative electrode. Thus, when a negative
electrode containing composite carbon material 2a doped with
lithium ion is combined with a positive electrode, the voltage of
the nonaqueous lithium power storage element is increased and the
utilizable capacity of the positive electrode is increased.
Therefore, the capacitance and energy density of the obtained
nonaqueous lithium power storage element increases.
If the lithium ion doping amount per unit weight of composite
carbon material 2a is 50 mAh/g or greater, lithium ion will be
satisfactorily doped even at irreversible sites where lithium ion
in the composite carbon material 2a cannot be desorbed after once
being inserted, and therefore high energy density can be obtained.
As the doping amount increases, the negative electrode potential
decreases and the input/output characteristic, energy density and
durability increase.
If the lithium ion doping amount per unit weight of the composite
carbon material 2a is no greater than 700 mAh/g, side-effects of
lithium metal deposition and the like will be less likely to
occur.
The BET specific surface area of the graphite material used in
composite carbon material 2a is preferably 0.5 m.sup.2/g to 80
m.sup.2/g, more preferably 1 m.sup.2/g to 70 m.sup.2/g and even
more preferably 1.5 m.sup.2/g to 60 m.sup.2/g. If the BET specific
surface area of the graphite material used in composite carbon
material 2a is within this range, it will be possible to adjust the
BET specific surface area of composite carbon material 2a to within
the range specified above.
The mean particle diameter of the graphite material used in
composite carbon material 2a is preferably 1 .mu.m to 10 .mu.m and
more preferably 2 .mu.m to 8 .mu.m. If the mean particle diameter
of the graphite material used in composite carbon material 2a is in
the range of 1 .mu.m to 10 .mu.m, it will be possible to adjust the
mean particle diameter of composite carbon material 2a to within
the range specified above.
A carbonaceous material precursor to be used as a starting material
for composite carbon material 2a is a solid, liquid or
solvent-soluble organic material that allows the carbonaceous
material to be composited with a graphite material by heat
treatment. The carbonaceous material precursor may be, for example,
pitch, mesocarbon microbeads, coke or a synthetic resin such as a
phenol resin, for example. Among such carbonaceous material
precursors, the use of inexpensive pitch is preferred in terms of
production cost. Pitch is largely classified as petroleum-based
pitch or coal-based pitch. Examples of petroleum-based pitch
include crude oil distillation residue, fluid catalytic cracking
residue (decant oil and the like), bottom oil from thermal
crackers, and ethylene tar obtained during naphtha cracking.
(Optional Components)
The negative electrode active material layer of this embodiment may
also contain optional components such as a conductive filler,
binder and dispersion stabilizer, as necessary, in addition to the
negative electrode active material.
The type of conductive filler is not particularly restricted, and
examples include acetylene black, Ketchen black and vapor grown
carbon fibers. The amount of conductive filler used is preferably 0
parts by weight to 30 parts by weight, more preferably 0 parts by
weight to 20 parts by weight and even more preferably 0 parts by
weight to 15 parts by weight, with respect to 100 parts by weight
of the negative electrode active material.
The binder is not particularly restricted, and for example, PVdF
(polyvinylidene fluoride), PTFE (polytetrafluoroethylene),
polyimide, latex, styrene-butadiene copolymer, fluorine rubber or
an acrylic copolymer may be used. The amount of binder used is
preferably 1 part by weight to 30 parts by weight, more preferably
2 parts by weight to 27 parts by weight and even more preferably 3
parts by weight to 25 parts by weight, with respect to 100 parts by
weight of the negative electrode active material. If the amount of
binder used is 1 part by weight or greater, adequate electrode
strength will be exhibited. If the amount of binder used is no
greater than 30 parts by weight, a high input/output characteristic
will be exhibited without inhibiting movement of lithium ions into
the negative electrode active material.
The dispersion stabilizer is not particularly restricted, and for
example, PVP (polyvinylpyrrolidone), PVA (polyvinyl alcohol) or
cellulose derivatives may be used. The amount of dispersion
stabilizer used is preferably 0 parts by weight to 10 parts by
weight, with respect to 100 parts by weight of the negative
electrode active material. If the amount of dispersion stabilizer
used is no greater than 10 parts by weight, a high input/output
characteristic will be exhibited without inhibiting movement of
lithium ions into the negative electrode active material.
[Negative Electrode Power Collector]
The material composing the negative electrode power collector of
this embodiment is preferably a metal foil with high electron
conductivity, and with resistance to degradation by elution into
the nonaqueous electrolytic solution or reaction with the
electrolyte or ion. There are no particular restrictions on such
metal foils, and examples include aluminum foils, copper foils,
nickel foils and stainless steel foils. The negative electrode
power collector in the nonaqueous lithium power storage element of
this embodiment is preferably a copper foil.
The metal foil may be a common metal foil without ruggedness or
through-holes, or it may be a metal foil having ruggedness formed
by embossing, chemical etching, electrolytic deposition or
blasting, or it may be a metal foil having through-holes, such as
an expanded metal, punching metal or etching foil.
Among these, the negative electrode power collector of this
embodiment is preferably a metal foil without through-holes. Having
no through-holes is cheaper in terms of production cost and
facilitates thin-film formation, and can thus contribute to higher
energy density, while also lowering the power collector resistance
and allowing a high input/output characteristic to be obtained.
The thickness of the negative electrode power collector is not
particularly restricted so long as it allows the shape and strength
of the negative electrode to be maintained, but 1 to 100 .mu.m, for
example, is preferred. Incidentally, when the negative electrode
power collector has through-holes or ruggedness, the thickness of
the negative electrode power collector is measured based on the
sections where no through-holes or ruggedness are present.
[Production of Negative Electrode]
The negative electrode comprises a negative electrode active
material layer on one or both sides of a negative electrode power
collector. Typically, the negative electrode active material layer
is anchored to one or both sides of the negative electrode power
collector.
The negative electrode can be produced by a known electrode
production technique for lithium ion batteries or electrical double
layer capacitors. For example, different materials containing a
negative electrode active material may be dispersed and dissolved
in water or an organic solvent to prepare a slurry-like coating
solution, and the coating solution coated onto one or both sides of
a negative electrode power collector to form a coating film, which
is dried to obtain a negative electrode. The obtained negative
electrode may also be pressed to adjust the film thickness or bulk
density of the negative electrode active material layer. As an
alternative method, various materials containing negative electrode
active materials may also be dry-mixed without using a solvent, and
the obtained mixture press-molded and then attached to a negative
electrode power collector using a conductive adhesive.
The coating solution may also be prepared by dry blending all or a
portion of each of the starting materials containing the negative
electrode active material, and then adding water or an organic
solvent, and/or a liquid or slurry-like substance comprising a
binder or dispersion stabilizer dissolved or dispersed in them. The
coating solution may also be prepared by adding various starting
powders containing the negative electrode active material, to a
liquid or slurry-like substance comprising a binder or dispersion
stabilizer dissolved or dispersed in water or an organic
solvent.
The method of dissolution or dispersion is not particularly
restricted, and a disperser such as a homodisperser or multiscrew
disperser, planetary mixer, thin-film spinning high-speed mixer or
the like, may be suitably used. In order to obtain a coating
solution in a satisfactorily dispersed state, it is preferred for
the dispersion to be at a circumferential speed of 1 m/s to 50 m/s.
It is preferred if the circumferential speed is 1 m/s or greater,
because this will allow each material to satisfactorily dissolve or
disperse. It is also preferred if the circumferential speed is no
greater than 50 m/s, because each material will be unlikely to be
broken down by heat or shear force during dispersion, and
reaggregation will be reduced.
The viscosity (.eta.b) of the coating solution of the negative
electrode is preferably 1,000 mPas to 20,000 mPas, more preferably
1,500 mPas to 10,000 mPas and even more preferably 1,700 mPas to
5,000 mPas. If the viscosity (.eta.b) of the coating solution of
the negative electrode is 1,000 mPas or higher, liquid dripping
during formation of the coating film will be suppressed, and the
coating film width and thickness can be satisfactorily controlled.
If the viscosity (.eta.b) of the coating solution of the negative
electrode is no higher than 20,000 mPas, there will be less
pressure loss in the flow channel of the coating solution when a
coating machine is used, allowing stable coating to be carried out,
and allowing control to less than the prescribed coating film
thickness.
The TI value (thixotropy index value) of the coating solution of
the negative electrode is preferably 1.1 or greater, more
preferably 1.2 or greater and even more preferably 1.5 or greater.
If the TI value of the coating solution of the negative electrode
is 1.1 or greater, it will be possible to satisfactorily control
the coating film width and thickness.
The method of forming a coating film of the negative electrode is
not particularly restricted, and a coating machine such as a die
coater, comma coater, knife coater or gravure coating machine may
be suitably used. The coating film may be formed by monolayer
coating or by multilayer coating. The coating speed is preferably
0.1 m/min to 100 m/min, more preferably 0.5 m/min to 70 m/min and
even more preferably 1 m/min to 50 m/min. If the coating speed is
0.1 m/min or greater then stable coating will be possible, and if
it is no greater than 100 m/min then coating precision can be
adequately ensured.
The method of drying the coating film of the negative electrode is
not particularly restricted, and a drying method such as hot air
drying or infrared ray (IR) drying may be suitably employed. Drying
of the coating film may be drying at a single temperature, or it
may be drying while varying the temperature in different stages.
Several drying methods may also be used in combination for drying.
The drying temperature is preferably 25.degree. C. to 200.degree.
C., more preferably 40.degree. C. to 180.degree. C. and even more
preferably 50.degree. C. to 160.degree. C. If the drying
temperature is 25.degree. C. or higher, it will be possible to
adequately volatilize off the solvent in the coating film. If the
drying temperature is no higher than 200.degree. C., it will be
possible to reduce cracking of the coating film by rapid
volatilization of the solvent or maldistribution of the binder by
migration, and oxidation of the negative electrode power collector
or negative electrode active material layer.
The method of pressing the negative electrode is not particularly
restricted, and a pressing machine such as a hydraulic press or
vacuum pressing machine may be suitably used. The film thickness,
bulk density and electrode strength of the negative electrode
active material layer can be adjusted by the pressing pressure, the
gap, and the surface temperature of the pressed portion, as
described below.
The pressing pressure is preferably 0.5 kN/cm to 20 kN/cm, more
preferably 1 kN/cm to 10 kN/cm and even more preferably 2 kN/cm to
7 kN/cm. If the pressing pressure is 0.5 kN/cm or greater, it will
be possible to adequately increase the electrode strength. If the
pressing pressure is no greater than 20 kN/cm, distortion or
wrinkling will be unlikely to occur in the negative electrode, and
adjustment of the negative electrode active material layer to the
desired film thickness and bulk density will be easier.
The gap between the press rolls may be set to a desired value
depending on the film thickness of the dried negative electrode, so
that the desired film thickness and bulk density of the negative
electrode active material layer is obtained.
The pressing speed may also be set to the desired speed, so as to
reduce distortion and wrinkling in the negative electrode. The
surface temperature of the pressed portion may be room temperature,
or it may be heated instead, if necessary.
In the case of heating, the lower limit for the surface temperature
of the pressed portion is preferably at least the melting point of
the binder -60.degree. C., more preferably at least the melting
point of the binder -45.degree. C., and even more preferably at
least the melting point of the binder -30.degree. C. The upper
limit for the surface temperature of the pressed portion in the
case of heating is also preferably no higher than the melting point
of the binder used +50.degree. C., more preferably no higher than
the melting point of the binder +30.degree. C., and even more
preferably no higher than the melting point of the binder
+20.degree. C. For example, when PVdF (polyvinylidene fluoride:
melting point=150.degree. C.) is used as the binder, heating of the
surface of the pressed portion is to preferably between 90.degree.
C. and 200.degree. C., more preferably between 105.degree. C. and
180.degree. C. and even more preferably between 120.degree. C. and
170.degree. C. When a styrene-butadiene copolymer (melting
point=100.degree. C.) is used as the binder, heating of the surface
of the pressed portion is to preferably between 40.degree. C. and
150.degree. C., more preferably between 55.degree. C. and
130.degree. C. and even more preferably between 70.degree. C. and
120.degree. C.
The melting point of the binder can be determined by the
endothermic peak position in DSC (Differential Scanning
Calorimetry). For example, using a "DSC7" differential scanning
calorimeter by Perkin-Elmer, 10 mg of sample resin is set in the
measuring cell and the temperature is increased from 30.degree. C.
to 250.degree. C. at a temperature-elevating rate of 10.degree.
C./min, in a nitrogen gas atmosphere, the melting point being the
endothermic peak temperature during the temperature elevation.
Pressing may also be carried out multiple times while varying the
conditions including the pressing pressure, gap, speed, and pressed
portion surface temperature.
The film thickness of the negative electrode active material layer
is preferably 5 .mu.m to 100 .mu.m, for each side of the negative
electrode power collector. The lower limit for the film thickness
of the negative electrode active material layer is more preferably
7 .mu.m or greater and even more preferably 10 .mu.m or greater.
The upper limit for the film thickness of the negative electrode
active material layer is more preferably no greater than 80 .mu.m
and even more preferably no greater than 60 .mu.m. If the film
thickness of the negative electrode active material layer is 5
.mu.m or greater, the coatability will be excellent with less
tendency to produce streaks during coating of the negative
electrode active material layer. If the film thickness of the
negative electrode active material layer is no greater than 100
.mu.m, high energy density can be exhibited, due to reduced volume
of the nonaqueous lithium power storage element. The film thickness
of the negative electrode active material layer, when the negative
electrode power collector has through-holes or ruggedness, is the
mean value of the film thickness of the negative electrode active
material layer for each side at the sections of the negative
electrode power collector without through-holes or ruggedness.
The bulk density of the negative electrode active material layer is
preferably 0.30 g/cm.sup.3 to 1.8 g/cm.sup.3, more preferably 0.40
g/cm.sup.3 to 1.5 g/cm.sup.3 and even more preferably 0.45
g/cm.sup.3 to 1.3 g/cm.sup.3. If the bulk density of the negative
electrode active material layer is 0.30 g/cm.sup.3 or greater,
sufficient strength can be obtained and sufficient conductivity can
be exhibited between the negative electrode active materials. If
the bulk density of the negative electrode active material layer is
1.8 g/cm.sup.3 or lower, it will be possible to ensure pores
through which the ions can be sufficiently diffused in the negative
electrode active material layer.
The BET specific surface area per unit volume of the negative
electrode active material layer is preferably 1 m.sup.2/cc to 50
m.sup.2/cc, more preferably 2 m.sup.2/cc to 40 m.sup.2/cc, even
more preferably 3 m.sup.2/cc to 35 m.sup.2/cc, yet more preferably
4 m.sup.2/cc to 30 m.sup.2/cc and most preferably 5 m.sup.2/cc to
20 m.sup.2/cc.
If the BET specific surface area per unit volume of the negative
electrode active material layer is 1 m.sup.2/cc or greater, the
reaction sites between the lithium ions in the nonaqueous
electrolytic solution and the negative electrode active material
layer can be adequately increased per unit volume of the negative
electrode active material layer, and therefore the nonaqueous
lithium power storage element using it can exhibit a high
input/output characteristic and high-load charge/discharge cycle
characteristic. On the other hand, if the BET specific surface area
per unit volume of the negative electrode active material layer is
no greater than 50 m.sup.2/cc, excessive reductive decomposition of
the nonaqueous electrolytic solution in the negative electrode
active material layer can be suppressed, and therefore a nonaqueous
lithium power storage element employing it can exhibit a high
high-load charge/discharge cycle characteristic.
The mean pore size of the negative electrode active material layer
is preferably 2 nm to 20 nm, more preferably 3 nm to 18 nm, even
more preferably 3.5 nm to 16 nm and most preferably 4 nm to 15
nm.
If the mean pore size of the negative electrode active material
layer is 2 nm or greater, there will be more pores in the negative
electrode active material layer having sizes larger than lithium
ion solvated in the nonaqueous electrolytic solution (approximately
0.9 nm to 1.2 nm), and therefore diffusion of the solvated lithium
ion in the negative electrode active material layer will be
satisfactory, and a nonaqueous lithium power storage element using
it can exhibit a high input/output characteristic. On the other
hand, if the mean pore size of the negative electrode active
material layer is no larger than 20 nm, the bulk density of the
negative electrode active material layer can be sufficiently
increased, and therefore a nonaqueous lithium power storage element
employing it can exhibit high energy density.
There are no particular restrictions on the methods for adjusting
the BET specific surface area per unit volume of the negative
electrode active material layer and the mean pore size of the
negative electrode active material layer to within the ranges
specified above for this embodiment, and they can be adjusted by
the type of negative electrode active material in the negative
electrode active material layer, or the types of conductive filler
and binder, as well as their weight ratio in the negative electrode
active material layer. For example, they can be adjusted by using a
negative electrode active material or conductive filler having a
BET specific surface area of 1 m.sup.2/g or greater and a mean pore
size of 1.5 nm or greater, and using a binder having a linear
structure such as PVdF (polyvinylidene fluoride) that can easily
fill pores of 2 nm and smaller. They can also be adjusted by the
coverage of the coating film or accumulation due to reductive
decomposition of the nonaqueous electrolytic solution in the
negative electrode active material layer, which is adjusted by the
composition of the nonaqueous electrolytic solution and the
production conditions for the nonaqueous lithium power storage
element.
For the purpose of the present specification, the BET specific
surface area per unit volume of the negative electrode active
material layer, and the mean pore size of the negative electrode
active material layer, can be calculated by the following
methods.
The sample used for measurement may be the negative electrode
before it is incorporated into the nonaqueous lithium power storage
element (hereunder also referred to as "negative electrode before
use"), or it may be the negative electrode incorporated in the
nonaqueous lithium power storage element (hereunder also referred
to as "negative electrode after use").
When the negative electrode incorporated in the nonaqueous lithium
power storage element is to be used as the measuring sample, the
following method, for example, is preferably used as pretreatment
of the measuring sample.
First, the nonaqueous lithium power storage element is disassembled
under an inert atmosphere such as argon, and the negative electrode
is removed. The removed negative electrode is immersed in a linear
carbonate (such as methyl ethyl carbonate or dimethyl carbonate),
the nonaqueous electrolytic solution and lithium salt are removed
and air-drying is carried out. Next, the following method (1), (2)
or (3) is preferably used.
(1) The obtained negative electrode is immersed in a mixed solvent
comprising methanol and isopropanol to inactivate the lithium ion
intercalated in the negative electrode active material, and
air-drying is carried out. Next, using vacuum drying or the like,
the linear carbonate and organic solvent in the obtained negative
electrode are removed to obtain a measuring sample.
(2) Using the obtained negative electrode as the working electrode
and metal lithium as the counter electrode and reference electrode,
they are immersed in the nonaqueous electrolytic solution under an
inert atmosphere such as argon, to fabricate an electrochemical
cell. A charger-discharger is used for adjustment of the obtained
electrochemical cell, so that the negative electrode potential (vs.
Li/Li.sup.+) is in the range of 1.5 V to 3.5 V. Next, the negative
electrode is removed from the electrochemical cell under an inert
atmosphere such as argon and immersed in a linear carbonate to
remove the nonaqueous electrolytic solution and lithium salt, and
air-drying is carried out. Next, vacuum drying or the like is used
to remove the linear carbonate in the obtained negative electrode,
to obtain a measuring sample.
(3) The obtained negative electrode may be used directly as the
measuring sample.
The volume V.sub.ano (cc) of the negative electrode active material
layer of the measuring sample obtained as described above is
measured, as shown in FIG. 1. The volume of the negative electrode
active material layer can be calculated by
V.sub.ano=S.sub.ano.times.t.sub.ano, where S.sub.ano is the
geometric area of the flat section of the measuring sample, when
the cross-section is on a horizontal plane with respect to the
direction of lamination of the negative electrode power collector
and negative electrode active material layer and the flat section
is on a plane crossing perpendicular to the horizontal surface, and
t.sub.ano is the total film thickness of the negative electrode
active material layer.
Using the obtained measuring sample, the adsorption/desorption
isotherm is measured with nitrogen or argon as the adsorbate. Using
the obtained isotherm on the adsorption side, the BET specific
surface area is calculated by the multipoint BET method or single
point BET method, and divided by V.sub.ano to calculate the BET
specific surface area per unit volume of the negative electrode
active material layer. The mean pore size of the negative electrode
active material layer is calculated by dividing the total pore
volume calculated by the measurement described above, by the BET
specific surface area.
The ratio r.sub.p/r.sub.a of the mean distance between the centers
of gravity of the voids obtained by SEM of a cross-section of the
negative electrode active material layer r.sub.p and the mean
particle diameter r.sub.a of the negative electrode active material
is preferably 0.10 to 1.10, more preferably 0.20 to 1.00, even more
preferably 0.25 to 0.80 and most preferably 0.30 to 0.60. If
r.sub.p/r.sub.a is 0.10 or greater, the sizes of the voids will be
sufficiently large with respect to the negative electrode active
material and a sufficient amount of nonaqueous electrolytic
solution will be able to be retained in the voids surrounding the
negative electrode active material, thereby allowing a nonaqueous
lithium power storage element to be obtained that exhibits a high
input/output characteristic and a high-load charge/discharge cycle
characteristic. If r.sub.p/r.sub.a is no greater than 1.10, the
nonaqueous electrolytic solution will be suitably dispersed around
the negative electrode active material, thereby allowing a
nonaqueous lithium power storage element to be obtained that
exhibits a high input/output characteristic and a high-load
charge/discharge cycle characteristic.
The BET specific surface area, mesopore volume and micropore volume
for this embodiment are the values determined by the following
respective methods. A sample is vacuum dried at 200.degree. C. for
a day and a night, and the adsorption/desorption isotherm is
measured using nitrogen as the adsorbate. Using the obtained
isotherm on the adsorption side, the BET specific surface area is
calculated by the multipoint BET method or single point BET method,
the mean pore size is calculated by dividing the total pore volume
per weight by the BET specific surface area, the mesopore volume is
calculated by the BJH method, and the micropore volume is
calculated by the MP method.
The BJH method is a method of calculation commonly used for
analysis of mesopores, and it was advocated by Barrett, Joyner,
Halenda et al. (E. P. Barrett, L. G. Joyner and P. Halenda, J. Am.
Chem. Soc., 73, 373(1951)).
The MP method is a method in which the "t-plot method" (B. C.
Lippens, J. H. de Boer, J. Catalysis, 4319(1965)) is utilized to
determine micropore volume, micropore area and micropore
distribution, and it is the method proposed by R. S. Mikhail,
Brunauer and Bodor (R. S. Mikhail, S. Brunauer, E. E. Bodor, J.
Colloid Interface Sci., 26, 45 (1968)).
The mean particle diameter for this embodiment is the particle
diameter at the point where, when the particle size distribution is
measured using a particle size distribution analyzer, and a
cumulative curve with 100% as the total volume is determined, the
cumulative curve is at 50% (that is, the 50% diameter (median
diameter)). The mean particle diameter can be measured using a
commercially available laser diffraction particle size distribution
analyzer.
The doping amount of lithium ion in the negative electrode active
material (mAh/g) of the nonaqueous lithium power storage element,
during shipping and after use, according to this embodiment, can be
determined in the following manner, for example.
First, after washing the negative electrode active material layer
of this embodiment with ethyl methyl carbonate or dimethyl
carbonate and air-drying it, it is extracted with a mixed solvent
comprising methanol and isopropanol, to obtain the extract and the
extracted negative electrode active material layer. The extraction
will typically be carried out in an Ar box at an environmental
temperature of 23.degree. C.
The lithium amounts in the extract obtained in this manner and the
extracted negative electrode active material layer are each
quantified using ICP-MS (Inductively Coupled Plasma-Mass
Spectrometry), for example, and the total is calculated to
determine the lithium ion doping amount in the negative electrode
active material. The obtained value may be compared to the amount
of negative electrode active material supplied for extraction, to
calculate the lithium ion doping amount (mAh/g).
The primary particle diameter according to this embodiment can be
obtained by a method in which the powder is photographed with an
electron microscope in several visual fields, the particle
diameters are calculated for 2,000 to 3,000 particles in the visual
fields using a fully automatic image processing device, and the
value of the arithmetic mean is recorded as the primary particle
diameter.
The degree of dispersion for this embodiment is the value
determined based on a dispersion evaluation test using a fineness
gauge conforming to JIS K5600. Specifically, a sufficient amount of
sample is allowed to flow into the tip of a fineness gauge having a
groove with the prescribed depth corresponding to the particle
size, through the deep part of the groove, and is allowed to
slightly spill over from the groove. Next, with the long side of a
scraper parallel to the widthwise direction of the gauge, and
placed with the blade edge in contact with the deep tip of the
groove of the fineness gauge, the scraper is held on the surface of
the gauge, the surface of the gauge is pulled at an even speed
perpendicular to the long side direction of the groove to a groove
depth of 0 for a period of 1 to 2 seconds, observation is made with
light irradiated at an angle of 20.degree. to 30 .degree. within 3
seconds after the pulling has ended, and the depth at which
particles appear in the groove of the fineness gauge is read
off.
The viscosity (.eta.b) and TI value for this embodiment are the
values determined by the following respective methods. First, an
E-type viscometer is used to determine the viscosity (.eta.a)
stabilized after measurement for 2 minutes or longer under
conditions with a temperature of 25.degree. C. and a shear rate of
2 s.sup.-1. Next, the viscosity (.eta.b) is determined as measured
under the same conditions except for changing the shear rate to 20
s.sup.-1. The viscosity values as obtained above are used to
calculate the TI value as: TI value=.eta.a/.eta.b. When increasing
the shear rate from 2 s.sup.-1 to 20 s.sup.-1, it may be increased
in a single stage, or the shear rate may be increased in stages
within the range specified above, while appropriately determining
the viscosity at each shear rate.
<Nonaqueous Electrolytic Solution>
The electrolytic solution for this embodiment is a lithium
ion-containing nonaqueous electrolytic solution. Specifically, the
nonaqueous electrolytic solution contains a nonaqueous solvent as
described below. The nonaqueous electrolytic solution preferably
comprises a lithium salt at a concentration of 0.5 mol/L or greater
based on the total volume of the nonaqueous electrolytic solution.
That is, the nonaqueous electrolytic solution contains lithium ion
as an electrolyte.
Examples of lithium salts include (LiN(SO.sub.2F).sub.2),
LiN(SO.sub.2CF.sub.3).sub.2, LiN(SO.sub.2C.sub.2F.sub.5).sub.2,
LiN(SO.sub.2CF.sub.3)(SO.sub.2C.sub.2F.sub.5),
LiN(SO.sub.2CF.sub.3)(SO.sub.2C.sub.2F.sub.4H),
LiC(SO.sub.2F).sub.3, LiC(SO.sub.2CF.sub.3).sub.3,
LiC(SO.sub.2C.sub.2F.sub.5).sub.3, LiCF.sub.3SO.sub.3,
LiC.sub.4F.sub.9SO.sub.3, LiPF.sub.6 and LiBF.sub.4, which may be
used alone or in mixtures of two more. The lithium salt also
preferably contains LiPF.sub.6 and/or LiN(SO.sub.2F).sub.2, since
these will allow high conductivity to be exhibited.
The lithium salt concentration in the nonaqueous electrolytic
solution is preferably 0.5 mol/L or greater, and more preferably in
the range of 0.5 to 2.0 mol/L. If the lithium salt concentration is
0.5 mol/L or greater, anions will be sufficiently present to allow
sufficiently high capacitance of the nonaqueous lithium power
storage element. The lithium salt concentration is preferably no
higher than 2.0 mol/L, because this can prevent precipitation of
the undissolved lithium salt in the nonaqueous electrolytic
solution and prevent the viscosity of the nonaqueous electrolytic
solution from becoming too high, and will help avoid lowering of
the conductivity or reduction in the output characteristic as
well.
The nonaqueous electrolytic solution of this embodiment preferably
comprises a cyclic carbonate and linear carbonate as nonaqueous
solvents. If the nonaqueous electrolytic solution comprises a
cyclic carbonate and a linear carbonate, this is advantageous from
the viewpoint of dissolving the lithium salt to the desired
concentration and exhibiting high lithium ion conductivity.
Examples of cyclic carbonates include alkylene carbonate compounds,
representative of which are ethylene carbonate, propylene carbonate
and butylene carbonate. An alkylene carbonate compound will
typically be unsubstituted. Linear carbonates include dialkyl
carbonate compounds, representative of which are dimethyl
carbonate, diethyl carbonate, methyl ethyl carbonate, dipropyl
carbonate and dibutyl carbonate. A dialkyl carbonate compound will
typically be unsubstituted.
The total content of the cyclic carbonate and linear carbonate is
preferably 50 weight % or greater and more preferably 65 weight %
or greater, and preferably no greater than 95 weight % and more
preferably no greater than 90 weight %, based on the total weight
of the nonaqueous electrolytic solution. If the total content of
the cyclic carbonate and linear carbonate is 50 weight % or greater
it will be possible to dissolve the desired concentration of
lithium salt, allowing high lithium ion conductivity to be
exhibited. If the total concentration of the cyclic carbonate and
linear carbonate is no greater than 95 weight %, the nonaqueous
electrolytic solution will be able to further comprise the
additives mentioned below.
The nonaqueous electrolytic solution of this embodiment may also
further comprise additives. The additives are not particularly
restricted, and for example, they include sultone compounds, cyclic
phosphazenes, acyclic fluoroethers, fluorinated cyclic carbonates,
cyclic carbonates, cyclic carboxylates, and cyclic acid anhydrides,
which may be used alone or in mixtures of two or more.
Examples of sultone compounds include sultone compounds represented
by the following formulas (5) to (7). Such sultone compounds can be
used alone or in mixtures of two or more.
##STR00001## {In formula (5), R.sup.11 to R.sup.16 represent
hydrogen atoms, halogen atoms, alkyl groups of 1 to 12 carbon atoms
or halogenated alkyl groups of 1 to 12 carbon atoms, and may be the
same or different; and n is an integer of 0 to 3.}
##STR00002## {In formula (6), R.sup.11 to R.sup.14 represent
hydrogen atoms, halogen atoms, alkyl groups of 1 to 12 carbon atoms
or halogenated alkyl groups of 1 to 12 carbon atoms, and may be the
same or different; and n is an integer of 0 to 3.}
##STR00003## {In formula (7), R.sup.11 to R.sup.16 represent
hydrogen atoms, halogen atoms, alkyl groups of 1 to 12 carbon atoms
or halogenated alkyl groups of 1 to 12 carbon atoms, and may be the
same or different.}
For this embodiment, from the viewpoint of minimal adverse effect
on resistance, and reducing decomposition of the nonaqueous
electrolytic solution at high temperature to minimize gas
generation, sultone compounds represented by formula (5) are
preferably 1,3-propanesultone, 2,4-butanesultone,
1,4-butanesultone, 1,3-butanesultone and 2,4-pentanesultone;
sultone compounds represented by formula (6) are preferably
1,3-propenesultone and 1,4-butenesultone; sultone compounds
represented by formula (7) are preferably 1,5,2,4-dioxadithiepane
2,2,4,4-tetraoxide; and other sultone compounds are preferably
methylenebis(benzenesulfonic acid),
methylenebis(phenylmethanesulfonic acid),
methylenebis(ethanesulfonic acid),
methylenebis(2,4,6,trimethylbenzenesulfonic acid) and
methylenebis(2-trifluoromethylbenzenesulfonic acid), with one or
more selected from among these groups being preferred.
The total content of sultone compounds in the nonaqueous
electrolytic solution of the nonaqueous lithium power storage
element of this embodiment is preferably 0.5 weight % to 15 weight
%, based on the total weight of the nonaqueous electrolytic
solution. If the total content of sultone compounds in the
nonaqueous electrolytic solution is 0.5 weight % or greater, it
will be possible to suppress decomposition of the nonaqueous
electrolytic solution at high temperature and to reduce gas
generation. If the total content of sultone compounds is no greater
than 15 weight %, on the other hand, it will be possible to lower
the ionic conductance of the nonaqueous electrolytic solution, and
to maintain a high input/output characteristic. The total content
of sultone compounds in the nonaqueous electrolytic solution of the
nonaqueous lithium power storage element is preferably 1 weight %
to 10 weight % and more preferably 3 weight % to 8 weight %, from
the viewpoint of obtaining both a high input/output characteristic
and high durability.
Examples of cyclic phosphazenes include
ethoxypentafluorocyclotriphosphazene,
diethoxytetrafluorocyclotriphosphazene and
phenoxypentafluorocyclotriphosphazene, and preferably one or more
selected from these groups is used.
The content of the cyclic phosphazene in the nonaqueous
electrolytic solution is preferably 0.5 weight % to 20 weight %
based on the total weight of the nonaqueous electrolytic solution.
If the cyclic phosphazene content is 0.5 weight % or greater, it
will be possible to minimize decomposition of the nonaqueous
electrolytic solution at high temperature and to reduce gas
generation. If the cyclic phosphazene content is no greater than 20
weight %, it will be possible to lower the ionic conductance of the
nonaqueous electrolytic solution, and to maintain a high
input/output characteristic. For these reasons, the cyclic
phosphazene content is preferably 2 weight % to 15 weight % and
more preferably 4 weight % to 12 weight %.
These cyclic phosphazenes may be used alone, or two or more may be
used in admixture.
Examples of acyclic fluoroethers include
HCF.sub.2CF.sub.2OCH.sub.2CF.sub.2CF.sub.2H,
CF.sub.3CFHCF.sub.2OCH.sub.2CF.sub.2CF.sub.2H,
HCF.sub.2CF.sub.2CH.sub.2OCH.sub.2CF.sub.2CF.sub.2H and
CF.sub.3CFHCF.sub.2OCH.sub.2CF.sub.2CFHCF.sub.3, among which
HCF.sub.2CF.sub.2OCH.sub.2CF.sub.2CF.sub.2H is preferred from the
viewpoint of electrochemical stability.
The content of the acyclic fluoroether is preferably 0.5 weight %
to 15 weight %, and more preferably 1 weight % to 10 weight %,
based on the total weight of the nonaqueous electrolytic solution.
If the acyclic fluoroether content is 0.5 weight % or higher, the
stability of the nonaqueous electrolytic solution against oxidative
decomposition will be increased and a nonaqueous lithium power
storage element with high durability during high temperature will
be obtained. If the acyclic fluoroether content is 15 weight % or
lower, on the other hand, the electrolyte salt solubility will be
kept satisfactory and high ionic conductance of the nonaqueous
electrolytic solution will be maintained, thus allowing a high
input/output characteristic to be exhibited.
The acyclic fluoroether used may be a single type or a mixture of
two or more types.
A fluorinated cyclic carbonate is preferably at least one selected
from the group consisting of fluoroethylene carbonate (FEC) and
difluoroethylene carbonate (dFEC), from the viewpoint of
compatibility with other nonaqueous solvents.
The content of the fluorinated cyclic carbonate is preferably 0.5
weight % to 10 weight %, and more preferably 1 weight % to 5 weight
%, with respect to the total weight of the nonaqueous electrolytic
solution. If the fluorinated cyclic carbonate is 0.5 weight % or
higher, it will be possible to form a satisfactory coating film on
the negative electrode, and reductive decomposition of the
nonaqueous electrolytic solution on the negative electrode will be
minimized, to obtain a nonaqueous lithium power storage element
with high durability at high temperature. If the fluorinated cyclic
carbonate content is 10 weight % or lower, on the other hand, the
electrolyte salt solubility will be kept satisfactory and high
ionic conductance of the nonaqueous electrolytic solution will be
maintained, thus allowing a high input/output characteristic to be
exhibited.
A fluorinated cyclic carbonate may be used alone, or two or more
may be used as a mixture.
The cyclic carbonate is preferably vinylene carbonate. The cyclic
carbonate content is preferably 0.5 weight % to 10 weight % and
more preferably 1 weight % to 5 weight %, with respect to the total
weight of the nonaqueous electrolytic solution. If the cyclic
carbonate content is 0.5 weight % or higher, it will be possible to
form a satisfactory coating film on the negative electrode, and
reductive decomposition of the nonaqueous electrolytic solution on
the negative electrode will be suppressed, to obtain a nonaqueous
lithium power storage element with high durability at high
temperature. If the cyclic carbonate content is 10 weight % or
lower, on the other hand, the electrolyte salt solubility will be
kept satisfactory and high ionic conductance of the nonaqueous
electrolytic solution will be maintained, thus allowing a high
input/output characteristic to be exhibited.
Examples of cyclic carboxylates include .gamma.-butyrolactone,
.gamma.-valerolactone, .gamma.-caprolactone and
.epsilon.-caprolactone, and preferably at least one selected from
these is used. Particularly preferred among these is
.gamma.-butyrolactone, from the viewpoint of improving the cell
characteristic due to improved lithium ion dissociation.
The cyclic carboxylate content is preferably 0.5 weight % to 15
weight % and more preferably 1 weight % to 5 weight %, with respect
to the total weight of the nonaqueous electrolytic solution. If the
cyclic carboxylate content is 0.5 weight % or higher, it will be
possible to form a satisfactory coating film on the negative
electrode, and reductive decomposition of the nonaqueous
electrolytic solution on the negative electrode will be suppressed,
to obtain a nonaqueous lithium power storage element with high
durability at high temperature. If the cyclic carboxylate content
is 5 weight % or lower, the electrolyte salt solubility will be
kept satisfactory and high ionic conductance of the nonaqueous
electrolytic solution will be maintained, thus allowing a high
input/output characteristic to be exhibited.
A cyclic carboxylate may be used alone, or two or more may be used
in admixture.
The cyclic acid anhydride is preferably one or more selected from
the group consisting of succinic anhydride, maleic anhydride,
citraconic anhydride and itaconic anhydride. Selection is most
preferably made from succinic anhydride and maleic anhydride, from
the viewpoint of ready industrial availability to reduce production
cost of the nonaqueous electrolytic solution, and from the
viewpoint of easier dissolution in the nonaqueous electrolytic
solution.
The content of the cyclic acid anhydride is preferably 0.5 weight %
to 15 weight %, and more preferably 1 weight % to 10 weight %, with
respect to the total weight of the nonaqueous electrolytic
solution. If the cyclic acid anhydride content is 0.5 weight % or
higher, it will be possible to form a satisfactory coating film on
the negative electrode, and reductive decomposition of the
nonaqueous electrolytic solution on the negative electrode will be
suppressed, to obtain a nonaqueous lithium power storage element
with high durability during periods of high temperature. If the
cyclic acid anhydride content is 10 weight % or lower, the
electrolyte salt solubility will be kept satisfactory and high
ionic conductance of the nonaqueous electrolytic solution will be
maintained, thus allowing a high input/output characteristic to be
exhibited.
These cyclic acid anhydrides may be used alone, or two or more may
be used in admixture.
<Separator>
The positive electrode precursor and negative electrode will
usually be laminated or wound via a separator, to form an electrode
laminated body or wound electrode comprising a positive electrode
precursor, negative electrode and separator.
The separator used may be a polyethylene microporous film or
polypropylene microporous film used in lithium ion secondary
batteries, or a cellulose nonwoven sheet used in electrical double
layer capacitors. A film composed of organic or inorganic
microparticles may also be laminated on one or both sides of these
separators. Organic or inorganic microparticles may also be
included inside a separator.
The thickness of the separator is preferably 5 .mu.m to 35 .mu.m.
The thickness of the separator is preferably 5 .mu.m or greater, as
this will tend to reduce self-discharge due to internal
microshorts. The thickness of the separator is also preferably no
greater than 35 .mu.m, as this will tend to result in a higher
input/output characteristic of the nonaqueous lithium power storage
element.
The thickness of a film composed of organic or inorganic
microparticles is preferably 1 .mu.m to 10 .mu.m. The thickness of
a film composed of organic or inorganic microparticles is
preferably 1 .mu.m or greater, as this will tend to reduce
self-discharge due to internal microshorts. The thickness of a film
composed of organic or inorganic microparticles is also preferably
no greater than 10 .mu.m, as this will tend to result in a higher
output characteristic of the nonaqueous lithium power storage
element.
<Production of Nonaqueous Lithium Power Storage Element>
The nonaqueous lithium power storage element of this embodiment is
typically constructed with an electrode laminated body or wound
electrode, as described below, housed in a casing together with a
nonaqueous electrolytic solution.
With the nonaqueous lithium power storage element of the invention,
a plurality of nonaqueous lithium power storage elements may be
connected in series or in parallel to create a power storage
module, for example. The nonaqueous lithium power storage element
and power storage module of the invention may be suitably utilized
in a power regenerating system of an automobile hybrid drive
system, a power load-leveling system for natural power generation
such as solar power generation or wind power generation, or in a
microgrid, an uninterruptable power source system for factory
production equipment or the like, a non-contact power supply system
designed for leveling of voltage fluctuation in microwave power
transmission or electrolytic resonance, or energy storage, or an
energy harvesting system designed for utilization of electric power
generated by vibration or the like, which are purposes that require
a high-load charge/discharge cycle characteristic.
The nonaqueous lithium power storage element of the invention is
preferably applied in a lithium ion capacitor or lithium ion
secondary battery, for example, where the effect of the invention
will be maximally exhibited.
[Assembly]
In the assembly step, for example, a positive electrode terminal
and negative electrode terminal are connected to a laminated body
formed by laminating a positive electrode precursor and negative
electrode cut into the shape of a sheet, via a separator, to
fabricate an electrode laminated body. Alternatively, a positive
electrode terminal and negative electrode terminal are connected to
a wound body formed by laminating or winding a positive electrode
precursor and negative electrode via a separator, to fabricate a
wound electrode. The shape of the wound electrode may be
cylindrical or flat.
The method of connecting the positive electrode terminal and
negative electrode terminal is not particularly restricted, and it
may be carried out by a method such as resistance welding or
ultrasonic welding.
[Casing]
The casing used may be a metal can or laminated package. A metal
can is preferably made of aluminum. The laminated package is
preferably a laminated film of a metal foil and a resin film, an
example of which is a laminated package comprising a three-layer
structure: outer layer resin film/metal foil/inner layer resin
film. The outer layer resin film serves to prevent damage to the
metal foil by contact, and a resin such as nylon or polyester may
be suitably used. The metal foil serves to prevent penetration of
moisture and gas, and a foil such as copper, aluminum or stainless
steel may be suitably used. The inner layer resin film serves to
protect the metal foil from the nonaqueous electrolytic solution
housed inside while also providing a melt seal during heat sealing
of the casing, and a polyolefin or acid-modified polyolefin may be
suitably used.
[Housing in Casing]
The dried electrode laminated body or wound electrode is preferably
stored in a casing, which is typically a metal can or laminated
package, and sealed, leaving only one of the openings. The method
of sealing the casing is not particularly restricted, but when
using a laminated package, a method such as heat sealing or impulse
sealing may be employed.
[Drying]
The electrode laminated body or wound electrode housed in the
casing is preferably dried to remove the residual solvent. The
drying method is not restricted, and drying may be carried out by
vacuum drying or the like. The residual solvent is preferably no
greater than 1.5 weight % based on the weight of the positive
electrode active material layer or negative electrode active
material layer. It is not preferred if the residual solvent is
greater than 1.5 weight %, because the solvent will remain in the
system and may impair the self-discharge property or cycle
characteristic.
[Filling, Impregnation and Sealing Step]
Upon completion of the assembly step, the electrode laminated body
or wound electrode housed in the casing is filled with a nonaqueous
electrolytic solution. After filling, impregnation is again carried
out and the positive electrode, negative electrode and separator
are preferably thoroughly wetted with the nonaqueous electrolytic
solution. If the nonaqueous electrolytic solution has not wetted at
least a portion of the positive electrode precursor, negative
electrode and separator, then in the lithium doping step described
below, lithium doping will proceed in a non-uniform manner,
resulting in increased resistance or lower durability of the
obtained nonaqueous lithium power storage element. The method of
impregnation is not particularly restricted, and for example, the
method used may be setting the filled electrode laminated body or
wound electrode in a pressure reduction chamber with the casing in
an opened state, using a vacuum pump to bring the interior of the
chamber to a reduced pressure state, and then restoring it to
atmospheric pressure. After impregnation, the electrode laminated
body or wound electrode having the casing in an open state may be
closed by sealing under reduced pressure.
[Lithium Doping Step]
In the lithium doping step, preferably a voltage is applied between
the positive electrode precursor and the negative electrode,
thereby decomposing the lithium compound in the positive electrode
precursor and releasing lithium ions, and reducing the lithium ions
at the negative electrode so that the negative electrode active
material layer is predoped with lithium ions.
During the lithium doping step, gas such as CO.sub.2 is generated
with oxidative decomposition of the lithium compound in the
positive electrode precursor. It is therefore preferable to provide
means for releasing the generated gas out of the casing during
application of the voltage. Examples of such means include a method
of applying a voltage with a portion of the casing in an open
state; and a method of applying voltage with appropriate outgassing
means such as a degassing valve or gas permeable film set
beforehand on a portion of the casing.
[Aging Step]
After the lithium doping step, the electrode laminated body or
wound electrode is preferably aged. In the aging step, the solvent
in the nonaqueous electrolytic solution is decomposed at the
negative electrode, and a lithium ion-permeable solid polymer
coating film is formed on the negative electrode surface.
The method of aging is not particularly restricted, and for
example, a method of reacting the solvent in the nonaqueous
electrolytic solution in a high-temperature environment may be
used.
[Degassing Step]
After the aging step, preferably degassing is further carried out
to reliably remove the gas remaining in the nonaqueous electrolytic
solution, positive electrode and negative electrode. Any gas
remaining in at least portions of the nonaqueous electrolytic
solution, positive electrode and negative electrode will interfere
with ion conduction, thus increasing the resistance of the obtained
nonaqueous lithium power storage element.
The method of degassing is not particularly restricted, and for
example, the method used may be setting the electrode laminated
body or wound electrode in a pressure reduction chamber with the
casing in an opened state, and using a vacuum pump to bring the
interior of the chamber to a reduced pressure state. After
degassing, the casing may be sealed to close the casing, and
fabricate a nonaqueous lithium power storage element.
<Evaluation of Properties of Nonaqueous Lithium Power Storage
Elements>
[Electrostatic Capacitance]
Throughout the present specification, the electrostatic capacitance
F (F) is the value obtained by the following method.
First, in a thermostatic bath set to 25.degree. C., the nonaqueous
lithium power storage element is subjected to constant-current
charge at the 2 C current value until 3.8 V is reached, and then
constant-voltage charge is carried out for 30 minutes with
application of a constant voltage of 3.8 V. Next, the capacitance
after constant-current discharge to 2.2 V at the 2 C current value
is recorded as Q. The obtained value of Q is used to determine the
value calculated by F=Q/(3.8-2.2), as the electrostatic capacitance
F (F).
[Electrical Energy]
Throughout the present specification, the electrical energy E (Wh)
is the value obtained by the following method.
The value calculated by F.times.(3.8.sup.2-2.2.sup.2)/2/3600, using
the electrostatic capacitance F (F) calculated by the method
described above, is the electrical energy E (Wh).
[Volume]
The volume of the nonaqueous lithium power storage element is the
volume of the portion of the electrode laminated body or wound
electrode in which the region where the positive electrode active
material layer and negative electrode active material layer are
stacked is housed by the casing.
For example, in the case of an electrode laminated body or wound
electrode housed by a laminated film, typically the region of the
electrode laminated body or wound electrode where the positive
electrode active material layer and negative electrode active
material layer are present is housed in a cup-shaped laminated
film. The volume (V.sub.1) of the nonaqueous lithium power storage
element is calculated by
V.sub.1=l.sub.1.times.w.sub.1.times.t.sub.1, using the outer length
(l.sub.1) and outer width (w.sub.1) of the cup-shaped section, and
the thickness (t.sub.1) of the nonaqueous lithium power storage
element including the laminated film.
In the case of an electrode laminated body or wound electrode
housed in a rectilinear metal can, the volume of the outer
dimensions of the metal can are simply used as the volume of the
nonaqueous lithium power storage element. That is, the volume
(V.sub.2) of the nonaqueous lithium power storage element is
calculated by V.sub.2=l.sub.2.times.w.sub.2.times.t.sub.2, based on
the outer length (l.sub.2) and outer width (w.sub.2), and outer
thickness (t.sub.2), of the rectilinear metal can.
Even in the case of a wound electrode housed in a cylindrical metal
can, the volume of the outer dimensions of the metal can is used as
the volume of the nonaqueous lithium power storage element. That
is, the volume (V.sub.3) of the nonaqueous lithium power storage
element is calculated by
V.sub.3=3.14.times.r.times.r.times.l.sub.3, using the outer radius
(r) and outer length (l.sub.3) of the bottom face or top face of
the cylindrical metal can.
[Energy Density]
Throughout the present specification, the energy density is the
value obtained by the formula E/V.sub.i (Wh/L), using the charge E
and volume V.sub.i (i=1, 2, 3) of the nonaqueous lithium power
storage element.
E/V.sub.i is preferably 15 or greater from the viewpoint of
exhibiting sufficient charge capacity and service capacity. It is
preferred if E/V.sub.i is 15 or greater, because it will be
possible to obtain a nonaqueous lithium power storage element with
excellent volume energy density, and therefore when a power storage
system using the nonaqueous lithium power storage element is used
in combination with an automobile engine, for example, the power
storage system can be installed in the narrow limited space inside
the automobile. The upper limit for E/V.sub.i is preferably no
greater than 50.
[Internal Resistance]
Throughout the present specification, the internal resistance Ra
(.OMEGA.) is the value obtained by the following method.
First, in a thermostatic bath set to 25.degree. C., the nonaqueous
lithium power storage element is subjected to constant-current
charge at the 20 C current value until 3.8 V is reached, and then
constant-voltage charge is carried out for 30 minutes with
application of a constant voltage of 3.8 V. Next, constant-current
discharge is carried out to 2.2 V with the 20 C current value, to
obtain a discharge curve (time-voltage). From the discharge curve,
with a voltage of E.sub.o at discharge time=0 seconds, obtained by
extrapolating by linear approximation from the voltage values at
discharge times of 2 seconds and 4 seconds, the value calculated
from voltage drop .DELTA.E=3.8-E.sub.o and Ra=.DELTA.E/(20 C
(current value A)) is the internal resistance Ra (.OMEGA.).
RaF is preferably no greater than 3.0, more preferably no greater
than 2.5 and even more preferably no greater than 2.2, from the
viewpoint of exhibiting sufficient charge capacity and service
capacity for high current. If RaF is no greater than 3.0, it will
be possible to obtain a nonaqueous lithium power storage element
having an excellent input/output characteristic. This is therefore
preferred since, by combining a power storage system using the
nonaqueous lithium power storage element with a high efficiency
engine, for example, it will be possible to adequately withstand
the high load applied to the nonaqueous lithium power storage
element. The lower limit for RaF is preferably 0.3 or greater.
[High-Load Charge/Discharge Cycle Test]
For the present specification, the resistance increase rate after a
high-load charge/discharge cycling test is measured by the
following method.
First, the nonaqueous lithium power storage element is subjected to
constant-current charge in a thermostatic bath set to 25.degree.
C., until reaching 3.8 V at the 300 C current value, and then
constant-current discharge is carried out until reaching 2.2 V at
the 300 C current value. High-load charge/discharge cycling is
repeated 60,000 times, and the internal resistance is measured by
the same method as for the internal resistance Ra (.OMEGA.)
described above, before start of the test and after completion of
the test, recording the internal resistance before start of the
test as Ra (.OMEGA.), and the internal resistance after completion
of the test as Rb (.OMEGA.). The resistance increase rate after the
high-load charge/discharge cycle test with respect to before start
of the test is calculated as Rb/Ra.
The resistance increase rate Rb/Ra after the high-load
charge/discharge cycle test is preferably no greater than 2.0, more
preferably no greater than 1.5 and even more preferably no greater
than 1.2. If the resistance increase rate after the high-load
charge/discharge cycle test is no greater than 2.0, the properties
of the nonaqueous lithium power storage element will be maintained
even with repeated charge/discharge. Consequently, it will be
possible to stably obtain an excellent input/output characteristic
for long periods, thus helping to prolong the usable life of the
nonaqueous lithium power storage element. The lower limit for Rb/Ra
is preferably 0.9 or greater.
EXAMPLES
Embodiments of the invention will now be explained in detail by
examples and comparative examples, with the understanding that
these examples and comparative examples are not limitative in any
way on the invention.
Example 1
[Preparation of Activated Carbon]
[Activated Carbon 1]
Crushed coconut shell carbide was placed in a small carbonizing
furnace and subjected to carbonization at 500.degree. C. for 3
hours under a nitrogen atmosphere, to obtain a carbide. The
obtained carbide was placed in an activating furnace, water vapor
in a heated state using a preheating furnace was introduced into
the activating furnace at 1 kg/h, and the temperature was increased
to 900.degree. C. over 8 hours for activation. The activated
carbide was cooled under a nitrogen atmosphere to obtain activated
carbon. The obtained activated carbon was flow-rinsed for 10 hours,
drained, and dried for 10 hours in an electrodesiccator held at
115.degree. C., and then it was pulverized for 1 hour with a ball
mill to obtain activated carbon 1.
A laser diffraction particle size distribution analyzer
(SALD-2000J) by Shimadzu Corp. was used to measure the mean
particle diameter of the activated carbon 1, which was found to be
4.2 .mu.m. Also, as a result of measuring the pore distribution of
activated carbon 1 using a pore distribution measuring apparatus by
Yuasa Ionics Co., Ltd. (AUTOSORB-1 AS-1-MP), the BET specific
surface area was 2,360 m.sup.2/g, the mesopore volume (V.sub.1) was
0.52 cc/g, the micropore volume (V.sub.2) was 0.88 cc/g and
V.sub.1/V.sub.2=0.59.
[Activated Carbon 2]
A phenol resin was placed in a firing furnace and subjected to
carbonization at 600.degree. C. for 2 hours under a nitrogen
atmosphere, and was then pulverized with a ball mill and sorted, to
obtain a carbide having a mean particle diameter of 7.0 .mu.m. The
obtained carbide was mixed with KOH at a weight ratio of 1:5, and
the mixture was placed in a firing furnace and heated at
800.degree. C. for 1 hour under a nitrogen atmosphere and
activated. The activated carbide was removed out and stirred and
rinsed for 1 hour in dilute hydrochloric acid adjusted to a
concentration of 2 mol/L, and then boiled and rinsed with distilled
water until the pH stabilized to between 5 and 6, after which it
was dried to produce activated carbon 2.
A laser diffraction particle size distribution analyzer
(SALD-2000J) by Shimadzu Corp. was used to measure the mean
particle diameter of the activated carbon 2, which was found to be
7.1 .mu.m. Also, as a result of measuring the pore distribution of
activated carbon 2 using a pore distribution measuring apparatus by
Yuasa Ionics Co., Ltd. (AUTOSORB-1 AS-1-MP), the BET specific
surface area was 3,627 m.sup.2/g, the mesopore volume (V.sub.1) was
1.50 cc/g, the micropore volume (V.sub.2) was 2.28 cc/g and
V.sub.1/V.sub.2=0.66.
[Activated Carbon 3]
A phenol resin was placed in a firing furnace and subjected to
carbonization at 600.degree. C. for 2 hours under a nitrogen
atmosphere, and was then pulverized with a ball mill and sorted, to
obtain a carbide having a mean particle diameter of 17.0 m. The
obtained carbide was mixed with KOH at a weight ratio of 1:5, and
the mixture was placed in a firing furnace and heated at
800.degree. C. for 1 hour under a nitrogen atmosphere and
activated. The activated carbide was removed out and stirred and
rinsed for 1 hour in dilute hydrochloric acid adjusted to a
concentration of 2 mol/L, and then boiled and rinsed with distilled
water until the pH stabilized to between 5 and 6, after which it
was dried to produce activated carbon 3.
A laser diffraction particle size distribution analyzer
(SALD-2000J) by Shimadzu Corp. was used to measure the mean
particle diameter of the activated carbon 3, which was found to be
17.0 .mu.m. Also, as a result of measuring the pore distribution of
activated carbon 2 using a pore distribution measuring apparatus by
Yuasa Ionics Co., Ltd. (AUTOSORB-1 AS-1-MP), the BET specific
surface area was 3,111 m.sup.2/g, the mesopore volume (V.sub.1) was
1.24 cc/g, the micropore volume (V.sub.2) was 2.02 cc/g and
V.sub.1/V.sub.2=0.62.
[Preparation of Positive Electrode Precursor]
Activated carbon 1 obtained above was used as the positive
electrode active material to produce a positive electrode
precursor.
After mixing 42.5 parts by weight of activated carbon 1, 45.0 parts
by weight of lithium carbonate having a mean particle diameter of
2.0 .mu.m, as a lithium compound, 3.0 parts by weight of Ketchen
black, 1.5 parts by weight of PVP (polyvinylpyrrolidone), 8.0 parts
by weight of PVdF (polyvinylidene fluoride) and NMP
(N-methylpyrrolidone), the mixture was dispersed using a
FILMIX.RTM. thin-film spinning high-speed mixer by Primix Corp.,
under conditions with a circumferential speed of 17 m/s, to obtain
a coating solution. The viscosity (.eta.b) and TI value of the
obtained coating solution were measured using a TVE-35H E-type
viscometer by Toki Sangyo Co., Ltd. As a result, the viscosity
(.eta.b) was 2,700 mPas and the TI value was 3.5. The degree of
dispersion of the obtained coating solution was measured using a
fineness gauge by Yoshimitsu Seiki Co. Ltd. As a result, the
granularity was 35 .mu.m. The coating solution was coated onto one
or both sides of an aluminum foil with a thickness of 15 .mu.m and
without through-holes, using a die coater by Toray Engineering Co.,
Ltd. under conditions with a coating speed of 1 m/s, and dried at a
drying temperature of 100.degree. C. to obtain a positive electrode
precursor (hereunder also referred to as "single-sided positive
electrode precursor" and "double-sided positive electrode
precursor", respectively). The obtained positive electrode
precursor was pressed using a roll press under conditions with a
pressure of 4 kN/cm and a pressed portion surface temperature of
25.degree. C.
[Preparation of Negative Electrode]
[Preparation Example for Negative Electrode 1]
A 150 g portion of commercially available coconut shell activated
carbon having a mean particle diameter of 3.0 .mu.m and a BET
specific surface area of 1,780 m.sup.2/g was placed into a
stainless steel mesh basket and set on a stainless steel vat
containing 270 g of coal pitch 1 (softening point: 50.degree. C.),
both were set in an electric furnace (furnace inner usable
dimension: 300 mm.times.300 mm.times.300 mm), and thermal reaction
was carried out to obtain composite carbon material 1. The heat
treatment was carried out under a nitrogen atmosphere, with
temperature increase to 600.degree. C. over a period of 8 hours,
and 4 hours of holding at the same temperature. This was followed
by natural cooling to 60.degree. C., after which the composite
carbon material 1 was removed out of the furnace.
The mean particle diameter and BET specific surface area of the
obtained composite carbon material 1 were measured by the same
methods as described above. As a result, the mean particle diameter
was 3.2 .mu.m and the BET specific surface area was 262 m.sup.2/g.
The weight ratio of coal pitch-derived carbonaceous material with
respect to activated carbon was 78%.
Composite carbon material 1 was then used as a negative electrode
active material to produce a negative electrode.
After mixing 85 parts by weight of composite carbon material 1, 10
parts by weight of acetylene black, 5 parts by weight of PVdF
(polyvinylidene fluoride) and NMP (N-methylpyrrolidone), the
mixture was dispersed using a FILMIX.RTM. thin-film spinning
high-speed mixer by Primix Corp., under conditions with a
circumferential speed of 15 m/s, to obtain a coating solution. The
viscosity (.eta.b) and TI value of the obtained coating solution
were measured using a TVE-35H E-type viscometer by Toki Sangyo Co.,
Ltd. As a result, the viscosity (.eta.b) was 2,789 mPas and the TI
value was 4.3. The coating solution was coated onto both sides of
an electrolytic copper foil with a thickness of 10 .mu.m and
without through-holes, using a die coater by Toray Engineering Co.,
Ltd. under conditions with a coating speed of 1 m/s, and dried at a
drying temperature of 85.degree. C. to obtain negative electrode 1
(hereunder also referred to as "double-sided negative electrode").
The obtained negative electrode 1 was pressed using a roll press
under conditions with a pressure of 4 kN/cm and a pressed portion
surface temperature of 25.degree. C. The film thickness of the
negative electrode active material layer of the obtained negative
electrode 1 was measured at 10 arbitrary locations of negative
electrode 1, using a Linear Gauge Sensor GS-551 by Ono Sokki Co.,
Ltd. The thickness of the copper foil was subtracted from the mean
value of the measured film thickness, to determine the film
thickness of the negative electrode active material layer of
negative electrode 1. The film thickness of the negative electrode
active material layers of negative electrode 1 was 40 .mu.m per
side.
[Preparation Example for Negative Electrodes 2 and 3]
Negative electrode active materials were prepared and evaluated in
the same manner as the preparation example for negative electrode
1, except that the preparation was with the base materials and
their parts by weight, the coal-based pitches and their parts by
weight, and the heat treatment temperatures shown in Table 1. Also,
negative electrodes were prepared and evaluated in the same manner
as the preparation example for negative electrode 1, except that
preparation was using the negative electrode active materials
obtained as described above, with the coating solutions listed in
Table 1. The results are shown in Table 1.
TABLE-US-00001 TABLE 1 Negative electrode active material Weight
Starting material Heat ratio of BET Base material Pitch 1 treatment
carbona- Mean specific Amount Amount ceous particle surface (parts
by (parts by temperature material diameter area Name Type weight)
weight) (.degree. C.) (%) (.mu.m) (m.sup.2/g) Negative Composite
Coconut 150 270 600 78 3.2 262 electrode 1 carbon shell material 1
activated carbon Negative Composite Carbon 150 150 1000 32 6.8 303
electrode 2 carbon nano- material 2 particles Negative Composite
Artificial 150 15 1000 2 4.9 6.1 electrode 3 carbon graphite 1
material 3 Coating solution Solid content Negative (parts by
weight) electrode active Negative Property material layer electrode
Viscosity film thickness active Acetylene .eta.b Adjusted per one
side material black PVdF (mp s) TI value (.mu.m) Negative 85 10 5
2,789 4.3 40 electrode 1 Negative 80 2 18 2,456 3.6 40 electrode 2
Negative 80 8 12 2,798 2.7 25 electrode 3
The starting materials in Table 1 are the following. Coconut shell
activated carbon: mean particle diameter=3.0 .mu.m, BET specific
surface area=1,780 m.sup.2/g Carbon nanoparticles: mean particle
diameter=5.2 .mu.m, BET specific surface area=859 m.sup.2/g,
primary particle size=20 nm Artificial graphite 1: mean particle
diameter=4.8 .mu.m, BET specific surface area=3.1 m.sup.2/g Pitch
1: coal-based pitch with softening point of 50.degree. C.
[Preparation of Nonaqueous Electrolytic Solution]
As an organic solvent there was used a mixed solvent of ethylene
carbonate (EC):methyl ethyl carbonate (EMC)=33:67 (volume ratio),
each electrolyte salt was dissolved so that the concentration ratio
of LiN(SO.sub.2F).sub.2 and LiPF.sub.6 was 25:75 (molar ratio) with
respect to the total nonaqueous electrolytic solution and the total
concentration of LiN(SO.sub.2F).sub.2 and LiPF.sub.6 was 1.2 mol/L,
and the obtained solution was used as a nonaqueous electrolytic
solution.
The concentrations of LiN(SO.sub.2F).sub.2 and LiPF.sub.6 in the
prepared nonaqueous electrolytic solution were 0.3 mol/L and 0.9
mol/L, respectively.
[Production of Nonaqueous Lithium Power Storage Element]
The obtained positive electrode precursor and negative electrode 1
were used to produce a plurality of nonaqueous lithium power
storage elements under the conditions described below.
[Assembly]
The obtained double-sided negative electrode and single-sided and
double-sided positive electrode precursors were cut to 10
cm.times.10 cm (100 cm.sup.2). Using a single-sided positive
electrode precursor for the uppermost side and lowermost side, 21
double-sided negative electrodes and 20 double-sided positive
electrode precursors were stacked, sandwiching microporous film
separators each with a thickness of 15 .mu.m between the negative
electrodes 1 and positive electrode precursors. Next, a negative
electrode terminal and positive electrode terminal were connected
to the negative electrodes 1 and positive electrode precursors,
respectively, by ultrasonic welding to obtain an electrode
laminated body. The electrode laminated body was housed in a casing
composed of an aluminum laminate package material, and the external
bodies 3 at the electrode terminal section and bottom section were
heat sealed under conditions with a temperature of 180.degree. C.,
a seal time of 20 sec and a seal pressure of 1.0 MPa. The sealed
body was vacuum dried under conditions with a temperature of
80.degree. C., a pressure of 50 Pa and a drying time of 60 hr.
[Filling, Impregnation and Sealing Step]
Approximately 80 g of the nonaqueous electrolytic solution was
injected into the electrode laminated body housed in the aluminum
laminate package material, in a dry air environment at atmospheric
pressure, a temperature of 25.degree. C. and a dew point of no
higher than -40.degree. C. Next, the aluminum laminate package
material housing the electrode laminated body was placed in a
pressure reduction chamber and the pressure was reduced from
atmospheric pressure to -87 kPa, after which it was restored to
atmospheric pressure and allowed to stand for 5 minutes. The step
of reducing the pressure from atmospheric pressure to -87 kPa and
then restoring to atmospheric pressure was subsequently repeated 4
times, and it was then allowed to stand for 15 minutes. The
pressure was again reduced from atmospheric pressure to -91 kPa,
and then restored to atmospheric pressure. The step of pressure
reduction and restoration to atmospheric pressure in the same
manner was repeated a total of 7 times (pressure reduction from
atmospheric pressure to -95, -96, -97, -81, -97, -97 and -97 kPa,
respectively). The electrode laminated body was impregnated with
the nonaqueous electrolytic solution by this procedure.
Next, the electrode laminated body housed in the aluminum laminate
package material and impregnated with the nonaqueous electrolytic
solution was placed in a pressure-reducing sealing machine, and
with pressure reduction to -95 kPa, it was sealed at 180.degree. C.
for 10 seconds at a pressure of 0.1 MPa to seal the aluminum
laminate package material and fabricate a nonaqueous lithium power
storage element.
[Lithium Doping Step]
The obtained nonaqueous lithium power storage element was subjected
to initial charging by a method of constant-current charging using
a charge/discharge apparatus (TOSCAT-3100U) by Toyo System Co.,
Ltd., in an environment of 25.degree. C. with a current value of 50
mA until reaching a voltage of 4.5 V, followed by constant-voltage
charge at 4.5 V continued for 48 hours, for lithium doping of the
negative electrode 1.
[Aging Step]
The lithium-doped nonaqueous lithium power storage element was
subjected to a constant-current/constant-voltage charge step, with
constant-current discharge in a 25.degree. C. environment at 150 mA
until reaching a voltage of 1.8 V, followed by constant-current
charge at 150 mA until reaching a voltage of 4.0 V, and further
constant-current discharge at 4.0 V for 5 hours. The nonaqueous
lithium power storage element was then stored in a 55.degree. C.
environment for 48 hours.
[Degassing Step]
A portion of the aluminum laminate package material of the aged
nonaqueous lithium power storage element was unsealed in a dry air
environment with a temperature of 25.degree. C. and a dew point of
-40.degree. C. Next, the nonaqueous lithium power storage element
was placed in a pressure reduction chamber, and a step of using a
diaphragm pump (N816.3KT.45.18) by KNF Co. for pressure reduction
over a period of 3 minutes from atmospheric pressure to -80 kPa,
followed by restoration to atmospheric pressure over a period of 3
minutes, was repeated 3 times. Next, the nonaqueous lithium power
storage element was placed in a pressure-reducing sealing machine,
and after pressure reduction to -90 kPa, it was sealed at
200.degree. C. for 10 seconds at a pressure of 0.1 MPa to seal the
aluminum laminate package material.
[Evaluation of Nonaqueous Lithium Power Storage Elements]
One of the obtained nonaqueous lithium power storage elements was
subjected to [Electrostatic capacitance and RaF measurement] and
[High-load charge/discharge cycle test], as described below. The
remaining nonaqueous lithium power storage element was used for
[Solid .sup.7Li-NMR measurement of positive electrode] and
[Measurement of mean particle diameter of lithium compound in
positive electrode] and [Quantitation of lithium compound], as
described below.
[Electrostatic Capacitance and RaF Measurement]
Each of the obtained nonaqueous lithium power storage elements was
used in the method described above in a thermostatic bath set to
25.degree. C., using a charge/discharge apparatus (5 V, 360 A) by
Fujitsu Telecom Networks, Ltd., to calculate the electrostatic
capacitance F and the internal resistance Ra at 25.degree. C., and
the energy density E/V.sub.1 and RaF were obtained. The results are
shown in Table 2.
[High-Load Charge/Discharge Cycle Test]
Each of the obtained nonaqueous lithium power storage elements was
used in the method described above in a thermostatic bath set to
25.degree. C., using a charge/discharge apparatus (5 V, 360 A) by
Fujitsu Telecom Networks, Ltd., for a high-load charge/discharge
cycle test, the internal resistance Rb after the high-load
charge/discharge cycle test was measured, and Rb/Ra was obtained.
The results are shown in Table 2.
[Solid 7Li-NMR Measurement of Positive Electrode]
The positive electrode of the obtained nonaqueous lithium power
storage element was used for solid .sup.7Li-NMR measurement of the
positive electrode active material layer.
First, the nonaqueous lithium power storage element produced as
described above was subjected to constant-current charge to 2.9 V
with a current of 50 mA, using a charge/discharge apparatus
(ACD-01) by Aska Electronic Co., Ltd., at an environmental
temperature of 25.degree. C., and then to
constant-current/constant-voltage charge with application of a
constant voltage of 2.9 V for 2 hours.
The positive electrode active material layer was then sampled under
an argon atmosphere. The nonaqueous lithium power storage element
was disassembled under an argon atmosphere, and the positive
electrode was removed. Next, the obtained positive electrode was
immersed in diethyl carbonate for 2 minutes or longer to remove the
lithium salt. After immersion once more in diethyl carbonate under
the same conditions, it was air-dried.
The positive electrode active material layer was then sampled from
the positive electrode.
The obtained positive electrode active material layer was used as a
sample for solid .sup.7Li-NMR measurement. Measurement was
performed by the single pulse method, using an ECA700 (.sup.7Li-NMR
resonance frequency: 272.1 MHz) by JEOL RESONANCE Inc. as the
measuring apparatus, in a room temperature environment, with a
magic-angle spinning rotational speed of 14.5 kHz and the
irradiation pulse width set to a 45.degree. pulse. The observation
range was -400 ppm to 400 ppm, and the number of points was 4,096.
Measurement was performed with repeated latency of 10 seconds and
3,000 seconds, using the same measuring conditions other than the
repeated latency, such as the same number of scans and receiver
gain, and an NMR spectrum was obtained. A 1 mol/L aqueous lithium
chloride solution was used as the shift reference, and the shift
position measured separately as an external standard was defined as
0 ppm. During measurement of the 1 mol/L aqueous lithium chloride
solution, the single pulse method was used with an irradiation
pulse width set to a 45.degree. pulse, without rotation of the
sample.
The value of b/a was calculated by the method described above, from
the solid .sup.7Li-NMR spectrum of the positive electrode active
material layer obtained by the method described above. The results
are shown in Table 2.
[Measurement of Mean Particle Diameter of Lithium Compound in
Positive Electrode]
The obtained nonaqueous lithium power storage element was
disassembled in an argon box with a dew point temperature of
-72.degree. C., and the positive electrode coated on both sides
with the positive electrode active material layer was cut out to a
size of 10 cm.times.5 cm and immersed in 30 g of a diethyl
carbonate solvent, occasionally moving the positive electrode with
a pincette, and was washed for 10 minutes. The positive electrode
was then removed out and air-dried for 5 minutes in an argon box,
and the positive electrode was immersed in 30 g of freshly prepared
diethyl carbonate solvent and washed for 10 minutes by the same
method as described above. The positive electrode was removed from
the argon box, and a vacuum dryer (DP33 by Yamato Scientific Co.,
Ltd.) was used for drying for 20 hours at a temperature of
25.degree. C. and a pressure of 1 kPa, to obtain a positive
electrode sample.
A small 1 cm.times.1 cm piece was cut out from the positive
electrode sample, and an SM-09020CP by JEOL Ltd. was used to create
a cross-section perpendicular to the in-plane direction of the
positive electrode sample using argon gas, under conditions with an
acceleration voltage of 4 kV and a beam diameter of 500 .mu.m. The
surface was then coated with gold by sputtering in a vacuum of 10
Pa. Next, the positive electrode surface was measured by SEM and
EDX with atmospheric exposure, under the conditions described
below.
(SEM-EDX measuring conditions)
Measuring apparatus: FE-SEM S-4700 Electrolytic emission scanning
electron microscope by Hitachi High-Technologies Corp. Acceleration
voltage: 10 kV Emission current: 10 .mu.A Measurement
magnification: 2,000x Electron beam incident angle: 90.degree.
X-ray take-off angle: 30.degree. Dead time: 15% Mapping elements:
C, O, F Measurement pixel count 256.times.256 pixels Measuring
time: 60 sec Number of scans: 50 The luminance and contrast were
adjusted so that the brightness had no pixel reaching the maximum
luminance, and the mean value of the brightness fell within the
range of 40% to 60% of luminance. (SEM-EDX Analysis)
The images obtained from SEM and EDX of the measured positive
electrode cross-section were subjected to image analysis by the
method described above using image analysis software (ImageJ), to
calculate the mean particle diameter X.sub.1 of the lithium
compound and the mean particle diameter Y.sub.1 of the positive
electrode active material. The results are shown in Table 2.
[Quantitation of Lithium Compound]
A positive electrode sample cut out to a size of 5 cm.times.5 cm
was immersed in methanol, and the vessel was capped and allowed to
stand for 3 days in an environment of 25.degree. C. The positive
electrode was then removed out and vacuum dried for 10 hours under
conditions of 120.degree. C., 5 kPa. The methanol solution after
washing was measured by GC/MS under conditions with a predrawn
calibration curve, and a diethyl carbonate abundance of less than
1% was confirmed. After then measuring the positive electrode
weight M.sub.0, the positive electrode sample was impregnated with
distilled water, and the vessel was capped and allowed to stand for
3 days in an environment of 45.degree. C. The positive electrode
sample was then removed out and vacuum dried for 12 hours under
conditions of 150.degree. C., 3 kPa. The distilled water after
washing was measured by GC/MS under conditions with a predrawn
calibration curve, and a methanol abundance of less than 1% was
confirmed. The positive electrode weight M.sub.1 was then measured,
a spatula, brush or bristles were used to remove the active
material layer on the positive electrode power collector, and the
weight M.sub.2 of the positive electrode power collector was
measured. The obtained M.sub.0, M.sub.1 and M.sub.2 values were
used to determine the content Z (wt %) of the lithium compound in
the positive electrode, by the method described above. The results
are shown in Table 2.
Examples 2 to 17 and Comparative Examples 1 to 4
Positive electrode precursors were prepared in the same manner as
Example 1, except that the positive electrode active materials, the
lithium compounds and their mean particle diameters, and the parts
by weight of the positive electrode active materials and lithium
compounds were as shown in Table 2. Nonaqueous lithium power
storage elements were prepared and evaluated in the same manner as
Example 1, except that these positive electrode precursors were
used and combined with the negative electrodes listed in Table 2.
The results are shown in Table 2.
Comparative Example 5
[Production of Positive Electrode Precursor]
After mixing 87.5 parts by weight of activated carbon 2, 3.0 parts
by weight of Ketchen black, 1.5 parts by weight of PVP
(polyvinylpyrrolidone), 8.0 parts by weight of PVDF (polyvinylidene
fluoride) and NMP (N-methylpyrrolidone), the mixture was dispersed
using a FILMIX.RTM. thin-film spinning high-speed mixer by Primix
Corp., under conditions with a circumferential speed of 17 m/s, to
obtain a coating solution. A positive electrode precursor was
obtained in the same manner as Example 1, except for using the
coating solution obtained above.
[Preparation and Evaluation of Nonaqueous Lithium Power Storage
Element]
Assembly, filling, impregnation and sealing of a nonaqueous lithium
power storage element were carried out in the same manner as
Example 1, except for using the obtained positive electrode
precursor, and a negative electrode comprising a metal lithium foil
corresponding to 211 mAh/g per unit weight of the negative
electrode active material, attached to the negative electrode
active material layer surface of the negative electrode 3.
Next, as the lithium doping step, the obtained nonaqueous lithium
power storage element was stored for 72 hours in a thermostatic
bath with an environmental temperature of 45.degree. C., for
ionization of the metal lithium and doping in the negative
electrode 3. The obtained nonaqueous lithium power storage element
was then subjected to an aging step and degassing step in the same
manner as Example 1, to produce a nonaqueous lithium power storage
element, which was evaluated. The results are shown in Table 2.
TABLE-US-00002 TABLE 2 Positive electrode precursor Mean Positive
particle electrode diameter of active Lithium Positive lithium
material compound electrode Positive electrode Lithium compound
(parts by (parts by Negative X.sub.1 active material compound
(.mu.m) weight) weight) electrode (.mu.m) Example 1 Activated
carbon 1 Lithium carbonate 2.0 42.5 45 Neg. electrode 1 1.2 Example
2 Activated carbon 1 Lithium carbonate 2.0 42.5 45 Neg. electrode 2
0.9 Example 3 Activated carbon 2 Lithium carbonate 1.0 67.5 20 Neg.
electrode 2 0.5 Example 4 Activated carbon 2 Lithium carbonate 4.0
42.5 45 Neg. electrode 2 2.3 Example 5 Activated carbon 2 Lithium
carbonate 6.0 32.5 55 Neg. electrode 2 5.1 Example 6 Activated
carbon 2 Lithium carbonate 1.0 67.5 20 Neg. electrode 3 0.2 Example
7 Activated carbon 2 Lithium carbonate 2.0 64.5 23 Neg. electrode 3
0.8 Example 8 Activated carbon 2 Lithium carbonate 2.0 62.5 25 Neg.
electrode 3 1.2 Example 9 Activated carbon 2 Lithium carbonate 4.0
42.5 45 Neg. electrode 3 2.5 Example 10 Activated carbon 2 Lithium
oxide 4.0 42.5 45 Neg. electrode 3 1.9 Example 11 Activated carbon
2 Lithium hydroxide 4.0 42.5 45 Neg. electrode 3 3.0 Example 12
Activated carbon 2 Lithium carbonate 6.0 35.5 52 Neg. electrode 3
3.5 Example 13 Activated carbon 2 Lithium carbonate 9.0 30.5 57
Neg. electrode 3 5.2 Example 14 Activated carbon 2 Lithium
carbonate 9.5 27.5 60 Neg. electrode 3 6.7 Example 15 Activated
carbon 3 Lithium carbonate 1.0 67.5 20 Neg. electrode 3 0.3 Example
16 Activated carbon 3 Lithium carbonate 6.0 49.5 38 Neg. electrode
3 4.3 Example 17 Activated carbon 3 Lithium carbonate 10.0 32.5 55
Neg. electrode 3 7.9 Comp. Activated carbon 2 Lithium carbonate 0.3
72.5 15 Neg. electrode 2 0.07 Example 1 Comp. Activated carbon 2
Lithium carbonate 12.5 17.5 70 Neg. electrode 2 10.3 Example 2
Comp. Activated carbon 2 Lithium carbonate 0.3 72.5 15 Neg.
electrode 3 0.07 Example 3 Comp. Activated carbon 2 Lithium
carbonate 12.5 17.5 70 Neg. electrode 3 11.2 Example 4 Comp.
Activated carbon 2 None -- 87.5 0 Neg. electrode 3 -- Example 5
Properties of nonaqueous lithium Positive electrode Solid power
storage element Y.sub.1 Z .sup.7Li-NMR E/V.sub.1 Ra F (.mu.m) (wt
%) b/a (Wh/L) (.OMEGA.F) Rd/Ra Example 1 4.0 17.7 1.82 16 1.32 1.08
Example 2 3.9 19.8 1.92 19 1.02 1.05 Example 3 6.9 1.2 1.05 24 0.69
1.72 Example 4 6.8 19.0 1.88 21 0.85 1.15 Example 5 7.1 43.1 3.72
20 1.02 1.10 Example 6 7.1 1.6 1.07 31 1.49 1.68 Example 7 6.7 2.6
1.12 31 1.55 1.25 Example 8 7.1 3.5 1.16 30 1.62 1.19 Example 9 6.9
20.0 1.93 26 1.74 1.13 Example 10 6.9 18.8 1.78 26 1.82 1.32
Example 11 7.0 21.3 2.10 26 1.79 1.10 Example 12 7.0 38.1 2.77 24
1.86 1.27 Example 13 6.9 42.3 3.59 22 1.92 1.31 Example 14 6.8 49.3
5.54 21 2.79 1.89 Example 15 16.7 1.3 1.06 43 1.98 1.82 Example 16
16.9 16.8 1.78 38 2.21 1.19 Example 17 16.7 44.2 3.88 32 2.67 1.33
Comp. 7.1 0.2 1.01 21 0.69 3.01 Example 1 Comp. 7.0 56.7 5.88 17
3.11 2.35 Example 2 Comp. 6.9 0.5 1.02 31 1.48 2.85 Example 3 Comp.
7.0 57.2 5.69 21 3.29 2.05 Example 4 Comp. 7.1 0.0 1.00 31 1.47
3.21 Example 5
From Examples 1 to 17 and Comparative Examples 1 to 5 it is seen
that by adjusting b/a to be in the range of
1.04.ltoreq.b/a.ltoreq.5.56, it is possible to obtain both low
resistance (i.e. a high input/output characteristic) and a high
high-load charge/discharge cycle characteristic.
Example 18
[Preparation of Positive Electrode Precursor]
After mixing 67.5 parts by weight of activated carbon 1 obtained in
Example 1, 20.0 parts by weight of lithium carbonate having a mean
particle diameter of 2.0 .mu.m, as a lithium compound, 3.0 parts by
weight of Ketchen black, 1.5 parts by weight of PVP
(polyvinylpyrrolidone), 8.0 parts by weight of PVdF (polyvinylidene
fluoride) and NMP (N-methylpyrrolidone), the mixture was dispersed
using a FILMIX.RTM. thin-film spinning high-speed mixer by Primix
Corp., under conditions with a circumferential speed of 17 m/s, to
obtain a coating solution. The viscosity (.eta.b) and TI value of
the obtained coating solution were measured using a TVE-35H E-type
viscometer by Toki Sangyo Co., Ltd. As a result, the viscosity
(.eta.b) was 2,820 mPas and the TI value was 3.2. The degree of
dispersion of the obtained coating solution was measured using a
fineness gauge by Yoshimitsu Seiki Co. As a result, the granularity
was 35 .mu.m. The coating solution was coated onto one or both
sides of an aluminum foil with a thickness of 15 .mu.m and without
through-holes, using a die coater by Toray Engineering Co., Ltd.
under conditions with a coating speed of 1 m/s, and dried at a
drying temperature of 100.degree. C. to obtain a positive electrode
precursor (hereunder also referred to as "single-sided positive
electrode precursor" and "double-sided positive electrode
precursor", respectively). The obtained positive electrode
precursor was pressed using a roll press under conditions with a
pressure of 4 kN/cm and a pressed portion surface temperature of
25.degree. C.
[Production of Nonaqueous Lithium Power Storage Element]
A plurality of nonaqueous lithium power storage elements were
produced in the same manner as Example 1, except for using the
positive electrode precursor obtained above and the negative
electrodes 1 listed in Table 1, under the conditions described
below under [Lithium doping step] and [Aging step].
[Lithium Doping Step]
The obtained nonaqueous lithium power storage element was subjected
to initial charging by a method of constant-current charging using
a charge/discharge apparatus (TOSCAT-3100U) by Toyo System Co.,
Ltd., in an environment of 25.degree. C. with a current value of 50
mA until reaching a voltage of 4.6 V, followed by constant-voltage
charge at 4.6 V continued for 72 hours, for lithium doping of the
negative electrode 1.
[Aging Step]
The lithium-doped nonaqueous lithium power storage element was
subjected to a constant-current/constant-voltage charge step, with
constant-current discharge in a 45.degree. C. environment at 100 mA
until reaching a voltage of 2.0 V, followed by constant-current
charge at 100 mA until reaching a voltage of 4.2 V, and further
constant-current discharge at 4.2 V for 72 hours.
[Evaluation of Nonaqueous Lithium Power Storage Elements]
One of the obtained nonaqueous lithium power storage elements was
subjected to [Electrostatic capacitance and RaF measurement] and
[High-load charge/discharge cycle test], as described above. The
results are shown in Table 3.
The remaining nonaqueous lithium power storage element was used to
carry out [Lithium amount in positive electrode] and [Quantitation
of compounds of formulas (1) to (3) in positive electrode active
material layer], below.
[Lithium Amount in Positive Electrode]
The positive electrode of the obtained nonaqueous lithium power
storage element was used for solid .sup.7Li-NMR measurement of the
positive electrode active material layer.
First, the nonaqueous lithium power storage element fabricated as
described above was subjected to constant-current charge to 2.9 V
with a current of 50 mA, using a charge/discharge apparatus
(ACD-01) by Aska Electronic Co., Ltd., at an environmental
temperature of 25.degree. C., and then to
constant-current/constant-voltage charge with application of a
constant voltage of 2.9 V for 2 hours.
The positive electrode active material layer was then sampled under
an argon atmosphere. The nonaqueous lithium power storage element
was disassembled under an argon atmosphere, and the positive
electrode was removed. Next, the obtained positive electrode was
immersed in diethyl carbonate for 2 minutes or longer to remove the
lithium salt. After immersion once more in diethyl carbonate under
the same conditions, it was air-dried. The positive electrode
active material layer was then sampled from the positive electrode,
and weighed.
The obtained positive electrode active material layer was used as a
sample for solid .sup.7Li-NMR measurement. Measurement was
performed by the single pulse method, using an ECA700 (.sup.7Li-NMR
resonance frequency: 272.1 MHz) by JEOL RESONANCE Inc. as the
measuring apparatus, in a room temperature environment, with a
magic-angle spinning rotational speed of 14.5 kHz and the
irradiation pulse width set to a 45.degree. pulse. A 1 mol/L
aqueous lithium chloride solution was used as the shift reference,
and the shift position measured separately as an external standard
was defined as 0 ppm. During measurement of the 1 mol/L aqueous
lithium chloride solution, the single pulse method was used with an
irradiation pulse width set to a 45.degree. pulse, without rotation
of the sample. During the measurement, a sufficient repeated
latency was taken between measurements, and each measurement was
performed with the repeated latency set to 3,000 seconds.
The lithium amount was calculated by the method described above,
from the solid .sup.7Li-NMR spectrum of the positive electrode
active material layer obtained by the method described above. The
results are shown in Table 3.
[Quantitation of Compounds of Formulas (1) to (3) in Positive
Electrode Active Material Layer]
After adjusting the nonaqueous lithium power storage element to 2.9
V, the nonaqueous lithium power storage element was disassembled in
an argon (Ar) box set in a room at 23.degree. C. and controlled to
a dew point of no higher than -90.degree. C. and an oxygen
concentration of no greater than 1 ppm, and the positive electrode
body was removed. The removed positive electrode body was immersed
and rinsed in dimethyl carbonate (DMC), and then vacuum dried in a
side box while maintaining a state of non-exposure to air.
The dried positive electrode was transferred from the side box to
an Ar box while maintaining a state of not being exposed to air,
and was immersed and extracted in heavy water to obtain a positive
electrode liquid extract. Analysis of the liquid extract was by (i)
IC and (ii) .sup.1H-NMR, and the abundance per unit weight of the
positive electrode active material layer (mol/g) for each compound
accumulated on the positive electrode body was determined by the
following formula 1: Abundance per unit weight (mol/g)=A.times.B/C
(1) from the concentration of each compound in the positive
electrode liquid extract A (mol/ml), the volume of heavy water used
for extraction B (ml) and the weight of active material of the
positive electrode active material layer used for extraction C
(g).
The active material weight in the positive electrode active
material layer used for extraction was determined by the following
method. The mixture (positive electrode active material layer) was
peeled off from the power collector of the positive electrode
remaining after heavy water extraction, and the peeled mixture was
rinsed with water and vacuum dried. The mixture obtained by vacuum
drying was washed with NMP or DMF. Next, the obtained positive
electrode active material layer was again vacuum dried, and weighed
to determine the weight of the positive electrode active material
layer used for extraction.
The positive electrode liquid extract was placed in a 3 mm.phi. NMR
tube (PN-002 by Shigemi Corp.) and inserted into a 5 mm.phi. NMR
tube (N-5 by Nihon Seimitsu Kagaku Co., Ltd.) containing
1,2,4,5-tetrafluorobenzene-added deuterated chloroform, and .sup.1H
NMR measurement was performed by the double tube method. By the 7.1
ppm (m, 2H) signal of 1,2,4,5-tetrafluorobenzene the integral of
each observed compound was calculated and normalized.
Deuterated chloroform containing dimethyl sulfoxide at known
concentration was placed in a 3 mm.phi. NMR tube (PN-002 by Shigemi
Corp.) and inserted into a 5 mm.phi. NMR tube (N-5 by Nihon
Seimitsu Kagaku Co., Ltd.) containing the same
1,2,4,5-tetrafluorobenzene-added deuterated chloroform as above,
and .sup.1H NMR measurement was performed by the double tube
method. In the same manner as above, by the 7.1 ppm (m, 2H) signal
of 1,2,4,5-tetrafluorobenzene the integral of the 2.6 ppm (s, 6H)
signal of dimethyl sulfoxide was calculated and normalized. The
concentration A of each compound in the positive electrode liquid
extract was determined based on the relationship between the
dimethyl sulfoxide concentration used and the integral.
Assignment for the .sup.1H NMR spectrum was as follows.
[For XOCH.sub.2CH.sub.2OX]
CH.sub.2 in XOCH.sub.2CH.sub.2OX: 3.7 ppm (s, 4H)
CH.sub.3OX: 3.3 ppm (s, 3H)
CH.sub.3 in CH.sub.3CH.sub.2OX: 1.2 ppm (t, 3H)
CH.sub.2O in CH.sub.3CH.sub.2OX: 3.7 ppm (q, 2H)
As mentioned above, the signal for CH.sub.2 in XOCH.sub.2CH.sub.2OX
(3.7 ppm) overlaps with the signal of CH.sub.2O in
CH.sub.3CH.sub.2OX (3.7 ppm), and therefore the amount of
XOCH.sub.2CH.sub.2OX was calculated by excluding the portion
corresponding to CH.sub.2O in CH.sub.3CH.sub.2OX calculated from
the signal for CH.sub.3 in CH.sub.3CH.sub.2OX (1.2 ppm).
Here, X is --(COO).sub.nLi or --(COO).sub.nR.sup.1 (where n is 0 or
1, and R.sup.1 is an alkyl group of 1 to 4 carbon atoms or a
halogenated alkyl group of 1 to 4 carbon atoms).
The amount of each of the compounds of formulas (1) to (3) in the
positive electrode active material layer was calculated from the
concentration of each compound in the extract determined by the
aforementioned analyses (i) and (ii), and also the volume of the
heavy water used for extraction and the weight of the positive
electrode active material layer used for extraction. The results
are shown in Table 3.
Examples 19 to 35 and Comparative Examples 7 to 9
Positive electrode precursors were produced in the same manner as
Example 18, except that the positive electrode active materials and
lithium compounds were as shown in Table 3. Nonaqueous lithium
power storage elements were produced and evaluated in the same
manner as Example 18, except that these positive electrode
precursors were used and combined with the negative electrodes
listed in Table 3, and the conditions for the aging step were as
listed in Table 3. The results are shown in Table 3.
Comparative Example 6
[Preparation of Positive Electrode Precursor]
After mixing 87.5 parts by weight of activated carbon 2 obtained in
Example 1, 3.0 parts by weight of Ketchen black, 1.5 parts by
weight of PVP (polyvinylpyrrolidone), 8.0 parts by weight of PVDF
(polyvinylidene fluoride) and NMP (N-methylpyrrolidone), the
mixture was dispersed using a FILMIX.RTM. thin-film spinning
high-speed mixer by Primix Corp., under conditions with a
circumferential speed of 17 m/s, to obtain a coating solution. A
positive electrode precursor was obtained in the same manner as
Example 18, except for using the coating solution obtained
above.
[Production and Evaluation of Nonaqueous Lithium Power Storage
Element]
Assembly, filling, impregnation and sealing of a nonaqueous lithium
power storage element were carried out in the same manner as
Example 18, except for using the obtained positive electrode
precursor, and a negative electrode comprising a metal lithium foil
corresponding to 1,150 mAh/g per unit weight of the negative
electrode active material, attached to the negative electrode
active material layer surface of the negative electrode 2.
Next, as the lithium doping step, the obtained nonaqueous lithium
power storage element was stored for 72 hours in a thermostatic
bath with an environmental temperature of 45.degree. C., for
ionization of the metal lithium and doping in the negative
electrode 2. A nonaqueous lithium power storage element was then
produced and evaluated in the same manner as Example 18, except
that the aging step for the obtained nonaqueous lithium power
storage element was carried out under the conditions listed in
Table 3. The results are shown in Table 3.
Comparative Example 8
A nonaqueous lithium power storage element was produced and
evaluated in the same manner as Comparative Example 6, except for
using a negative electrode comprising a metal lithium foil
corresponding to 211 mAh/g per unit weight of the negative
electrode active material, attached to the negative electrode
active material layer surface of negative electrode 3. The results
are shown in Table 3.
The results are summarized in Table 3 below.
TABLE-US-00003 TABLE 3 Positive electrode Total amount of
Properties of compounds nonaqueous Aging step Lithium of formulas
lithium power Positive electrode precursor Temper- Volt- amount (1)
to (3) storage element Positive electrode Lithium ature age Time
(mmol/ (.times.10.sup.-4 E/V.s- ub.1 Ra F Rb/ active material
compound Negative electrode (.degree. C.) (V) (hr) g) mol/g) (Wh/L)
(.OMEGA.F) Ra Example 18 Activated carbon 1 Lithium carbonate
Negative electrode 1 45 4.2 72 10.6 141.7 17 1.02 1.20 Example 19
Activated carbon 1 Lithium carbonate Negative electrode 2 45 4.2 72
11.1 149.3 20 0.92 1.16 Example 20 Activated carbon 2 Lithium
carbonate Negative electrode 2 0 4.2 10 1.1 2.10 23 0.69 1.72
Example 21 Activated carbon 2 Lithium carbonate Negative electrode
2 45 4.2 10 1.3 17.2 23 0.73 1.55 Example 22 Activated carbon 2
Lithium carbonate Negative electrode 2 75 4.2 10 1.4 58.2 23 0.79
1.52 Example 23 Activated carbon 2 Lithium carbonate Negative
electrode 2 0 4.2 72 11.5 72.1 23 0.82 1.35 Example 24 Activated
carbon 2 Lithium carbonate Negative electrode 2 45 4.2 72 11.8
156.6 23 0.85 1.15 Example 25 Activated carbon 2 Lithium carbonate
Negative electrode 2 75 4.2 72 12.0 255.2 23 0.89 1.21 Example 26
Activated carbon 2 Lithium carbonate Negative electrode 2 0 4.2 110
28.8 201.2 23 1.09 1.24 Example 27 Activated carbon 2 Lithium
carbonate Negative electrode 2 45 4.2 110 28.8 296.3 23 1.27 1.41
Example 28 Activated carbon 2 Lithium carbonate Negative electrode
3 45 4.2 10 1.3 14.5 31 1.49 1.76 Example 29 Activated carbon 2
Lithium carbonate Negative electrode 3 45 4.2 24 3.2 40.4 31 1.52
1.65 Example 30 Activated carbon 2 Lithium carbonate Negative
electrode 3 45 4.2 48 5.5 73.6 31 1.59 1.36 Example 31 Activated
carbon 2 Lithium carbonate Negative electrode 3 45 4.2 72 11.5
136.1 31 1.74 1.12 Example 32 Activated carbon 2 Lithium oxide
Negative electrode 3 45 4.2 72 12.1 141.9 31 1.81 1.18 Example 33
Activated carbon 2 Lithium hydroxide Negative electrode 3 45 4.2 72
10.9 122.2 31 1.75 1.15 Example 34 Activated carbon 2 Lithium
carbonate Negative electrode 3 45 4.2 96 22.0 261.9 31 1.88 1.25
Example 35 Activated carbon 2 Lithium carbonate Negative electrode
3 45 4.2 120 27.8 292.0 31 2.23 1.36 Comp. Activated carbon 2 None
Negative electrode 2 0 4.2 10 0.4 0.9 23 0.68 2.74 Example 6 Comp.
Activated carbon 2 Lithium carbonate Negative electrode 2 55 4.5
120 32.3 362.1 23 1.47 2.08 Example 7 Comp. Activated carbon 2 None
Negative electrode 3 0 4.2 10 0.7 1.5 31 1.50 2.57 Example 8 Comp.
Activated carbon 2 Lithium carbonate Negative electrode 3 55 4.5
120 33.1 341.2 31 3.11 2.10 Example 9
From Examples 18 to 35 and Comparative Examples 6 to 9, it is seen
that by adjusting the lithium amount, calculated by the peak area
from -40 ppm to 40 ppm in the solid .sup.7Li-NMR spectrum of the
positive electrode active material layer, to 1 mmol/g to 30 mmol/g,
it is possible to obtain both low resistance (that is, a high
input/output characteristic) and a high high-load charge/discharge
cycle characteristic.
Example 36
[Preparation of Positive Electrode Precursor]
After mixing 57.5 parts by weight of activated carbon 1 obtained in
Example 1, 30.0 parts by weight of lithium carbonate having a mean
particle diameter of 1.8 .mu.m, as a lithium compound, 3.0 parts by
weight of Ketchen black, 1.5 parts by weight of PVP
(polyvinylpyrrolidone), 8.0 parts by weight of PVdF (polyvinylidene
fluoride) and NMP (N-methylpyrrolidone), the mixture was dispersed
using a FILMIX.RTM. thin-film spinning high-speed mixer by Primix
Corp., under conditions with a circumferential speed of 17 m/s, to
obtain a coating solution. The viscosity (.eta.b) and TI value of
the obtained coating solution were measured using a TVE-35H E-type
viscometer by Toki Sangyo Co., Ltd., and as a result the viscosity
(.eta.b) was 2,590 mPas and the TI value was 2.8. The degree of
dispersion of the obtained coating solution was measured using a
fineness gauge by Yoshimitsu Seiki Co. The granularity was 35
.mu.m. The coating solution was coated onto one or both sides of an
aluminum foil with a thickness of 15 .mu.m and without
through-holes, using a die coater by Toray Engineering Co., Ltd.
under conditions with a coating speed of 1 m/s, and dried at a
drying temperature of 100.degree. C. to obtain a positive electrode
precursor (hereunder referred to as "single-sided positive
electrode precursor" and "double-sided positive electrode
precursor", respectively). The obtained positive electrode
precursor was pressed using a roll press under conditions with a
pressure of 4 kN/cm and a pressed portion surface temperature of
25.degree. C.
[Preparation of Negative Electrode]
[Preparation Example for Negative Electrode 4]
Artificial graphite 2 having a mean particle diameter of 9.7 .mu.m
and a BET specific surface area of 1.2 m.sup.2/g, used in an amount
of 150 g, was placed into a stainless steel mesh basket and set on
a stainless steel vat containing 15 g of coal pitch 2 (softening
point: 65.degree. C.), and both were set in an electric furnace
(furnace inner usable dimension: 300 mm.times.300 mm.times.300 mm).
This was increased in temperature to 1,250.degree. C. for 8 hours
under a nitrogen atmosphere, and kept at the same temperature for 4
hours for thermal reaction to obtain composite carbon material 4.
The obtained composite carbon material 4 was cooled to 60.degree.
C. by natural cooling, and then removed out from the electric
furnace.
The mean particle diameter and BET specific surface area of the
obtained composite carbon material 4 were measured by the same
methods as described above. The results are shown in Table 4.
Composite carbon material 4 was then used as a negative electrode
active material to produce negative electrode 4.
After mixing 80 parts by weight of composite carbon material 4, 8
parts by weight of acetylene black, 12 parts by weight of PVdF
(polyvinylidene fluoride) and NMP (N-methylpyrrolidone), the
mixture was dispersed using a FILMIX.RTM. thin-film spinning
high-speed mixer by Primix Corp., under conditions with a
circumferential speed of 15 m/s, to obtain a coating solution. The
viscosity (.eta.b) and TI value of the obtained coating solution
were measured using a TVE-35H E-type viscometer by Toki Sangyo Co.,
Ltd., and as a result the viscosity (.eta.b) was 2,674 mPas and the
TI value was 2.6. The coating solution was coated onto both sides
of an electrolytic copper foil with a thickness of 10 .mu.m and
without through-holes, using a die coater by Toray Engineering Co.,
Ltd. under conditions with a coating speed of 1 m/s, and dried at a
drying temperature of 85.degree. C. to obtain negative electrode 4
(hereunder also referred to as "double-sided negative electrode").
The obtained negative electrode 4 was pressed using a roll press.
The film thickness of the obtained negative electrode 4 was
measured at 10 arbitrary locations of negative electrode 4, using a
Linear Gauge Sensor GS-551 by Ono Sokki Co., Ltd. The thickness of
the copper foil was subtracted from the mean value of the measured
film thickness, to determine the film thickness of the negative
electrode active material layer of negative electrode 4. The film
thickness of the negative electrode active material layer of
negative electrode 4 was 20 .mu.m per side.
[Preparation Example for Negative Electrodes 5 to 13]
Negative electrode active materials were produced and evaluated in
the same manner as the preparation example for negative electrode
4, except that the base materials and their parts by weight, the
coal-based pitches and their parts by weight, and the heat
treatment temperatures were adjusted as shown in Table 4. Also,
negative electrodes 5 to 13 were produced and evaluated in the same
manner as the preparation example for negative electrode 4, except
that the negative electrode active materials listed in Table 4 were
used, and the coating solutions were adjusted to the coating
solution compositions listed in Table 4. The results are shown in
Table 4.
TABLE-US-00004 TABLE 4 Negative electrode active Negative electrode
active material Coating solution material Starting material Heat
Weight BET Negative layer Base material Pitch 2 treat- ratio of
Mean Specif- electrode Conductive filler film Amount Amount ment
carbona- particle ic active Amount PVdF thickness (parts (parts
temper- ceous diam- surface material (parts (parts per one by by
ature material eter area (parts by by by side Name Type weight)
weight) (.degree. C.) (weight %) (.mu.m) (m.sup.2/g) weight) Type
weight) weight) (.mu.m) Negative Composite Artificial 150 15 1250 2
9.8 1.5 80 Acetylene 8 12 20 electrode 4 carbon graphite black
material 4 2 Negative Composite Artificial 150 15 1250 2 9.8 1.5 80
Ketchen 8 12 20 electrode 5 carbon graphite black material 4 2
Negative Composite Artificial 150 15 1250 2 6.2 7.3 80 Acetylene 8
12 20 electrode 6 carbon graphite black material 5 3 Negative
Composite Artificial 150 15 1250 2 6.2 7.3 80 Ketchen 8 12 20
electrode 7 carbon graphite black material 5 3 Negative Composite
Artificial 150 50 1250 21 2.2 30.2 80 Ketchen 8 12 20 electrode 8
carbon graphite black material 6 4 Negative Composite Natural 150
15 1250 2 8.0 2.4 80 Acetylene 8 12 20 electrode 9 carbon graphite
black material 7 1 Negative Composite Natural 150 15 1250 2 3.2 7.9
80 Acetylene 8 12 20 electrode carbon graphite black 10 material 8
2 Negative Composite Natural 150 50 1250 22 1.5 45.2 80 Acetylene 8
12 20 electrode carbon graphite black 11 material 9 3 Negative
Composite High 150 30 1250 15 5.8 19.5 80 Acetylene 8 12 20
electrode carbon area-to- black 12 material 10 weight ratio
graphite 1 Negative Composite High 150 30 1250 17 5.2 49.3 80
Acetylene 8 12 20 electrode carbon area-to- black 13 material 11
weight ratio graphite 2
The starting materials in Table 4 are the following. Artificial
graphite 2: mean particle diameter=9.7 .mu.m, BET specific surface
area=1.2 m.sup.2/g Artificial graphite 3: mean particle
diameter=6.1 .mu.m, BET specific surface area=6.6 m.sup.2/g
Artificial graphite 4: mean particle diameter=2.1 .mu.m, BET
specific surface area=13.7 m.sup.2/g Natural graphite 1: mean
particle diameter=7.9 .mu.m, BET specific surface area=2.0
m.sup.2/g Natural graphite 2: mean particle diameter=3.1 .mu.m, BET
specific surface area=6.9 m.sup.2/g Natural graphite 3: mean
particle diameter=1.3 .mu.m, BET specific surface area=16.7
m.sup.2/g High specific surface area graphite 1: mean particle
diameter=5.5 .mu.m, BET specific surface area=27.7 m.sup.2/g High
specific surface area graphite 2: mean particle diameter=4.9 .mu.m,
BET specific surface area=58.9 m.sup.2/g Pitch 2: coal-based pitch
with softening point of 65.degree. C. [Production of Nonaqueous
Lithium Power Storage Element]
A plurality of nonaqueous lithium power storage elements were
produced in the same manner as Example 1, except for using the
positive electrode precursor obtained above and negative electrode
4, under the conditions described below under [Lithium doping step]
and [Aging step].
[Lithium Doping Step]
The obtained nonaqueous lithium power storage element was subjected
to initial charging by a method of constant-current charging using
a charge/discharge apparatus (TOSCAT-3100U) by Toyo System Co.,
Ltd., in an environment of 50.degree. C. with a current value of
150 mA until reaching a voltage of 4.5 V, followed by
constant-voltage charge at 4.5 V continued for 12 hours, for
lithium doping of the negative electrode 4.
[Aging Step]
The lithium-doped nonaqueous lithium power storage element was
subjected to a constant-current/constant-voltage charge step, with
constant-current discharge in a 25.degree. C. environment at 50 mA
until reaching a voltage of 2.2 V, followed by constant-current
charge at 50 mA until reaching a voltage of 4.0 V, and further
constant-current charge at 4.0 V for 30 hours.
[Evaluation of Nonaqueous Lithium Power Storage Elements]
One of the obtained nonaqueous lithium power storage elements was
subjected to [Electrostatic capacitance and RaF measurement] and
[High-load charge/discharge cycle test], as described above. The
results are shown in Table 5.
The remaining nonaqueous lithium power storage element was used for
[Solid .sup.7Li-NMR measurement of negative electrode], [Analysis
of negative electrode active material layer of negative electrode
after use], [Solid .sup.7Li-NMR measurement of positive electrode]
described below and [Measurement of mean particle diameter of
lithium compound in positive electrode] described above.
[Solid .sup.7Li-NMR Measurement of Negative Electrode]
Negative electrode 4 of the nonaqueous lithium power storage
element obtained above was used for solid .sup.7Li-NMR measurement
of the negative electrode active material layer.
First, the nonaqueous lithium power storage element produced as
described above was subjected to constant-current charge to 2.9 V
with a current of 50 mA, using a charge/discharge apparatus
(ACD-01) by Aska Electronic Co., Ltd., at an environmental
temperature of 25.degree. C., and then to
constant-current/constant-voltage charge with application of a
constant voltage of 2.9 V for 15 hours.
The negative electrode active material layer was then sampled under
an argon atmosphere. The nonaqueous lithium power storage element
was disassembled under an argon atmosphere, and the negative
electrode 4 was removed. Next, the obtained negative electrode 4
was immersed in diethyl carbonate for 2 minutes or longer to remove
the lithium salt. After immersion once more in diethyl carbonate
under the same conditions, it was air-dried. The negative electrode
active material layer was then sampled from negative electrode 4,
and weighed.
The obtained negative electrode active material layer was used as a
sample for solid .sup.7Li-NMR measurement. Measurement was
performed by the single pulse method, using an ECA700 (.sup.7Li-NMR
resonance frequency: 272.1 MHz) by JEOL RESONANCE Inc. as the
measuring apparatus, in a room temperature environment, with a
magic-angle spinning rotational speed of 14.5 kHz and the
irradiation pulse width set to a 45.degree. pulse. A 1 mol/L
aqueous lithium chloride solution was used as the shift reference,
and the shift position measured separately as an external standard
was defined as 0 ppm. During measurement of the 1 mol/L aqueous
lithium chloride solution, the single pulse method was used with an
irradiation pulse width set to a 45.degree. pulse, without rotation
of the sample.
In the solid .sup.7Li-NMR spectrum of the negative electrode active
material layer obtained by the method described above, the position
of the maximum peak in the spectral range of -10 ppm to 35 ppm was
16 ppm. Also, the amount of lithium per unit weight of the negative
electrode active material layer that had intercalated lithium ions
was calculated by the method described above, from the solid
.sup.7Li-NMR spectrum of the obtained negative electrode active
material layer. The results are shown in Table 5.
[Analysis of Negative Electrode Active Material Layer of Negative
Electrode after Use]
Negative electrode 4 of the nonaqueous lithium power storage
element obtained as described above was used to measure the BET
specific surface area per unit volume of the negative electrode
active material layer of the negative electrode after use, and the
mean pore size of the negative electrode active material layer.
First, the nonaqueous lithium power storage element produced as
described above was subjected to constant-current charge to 2.9 V
with a current of 50 mA, using a charge/discharge apparatus
(ACD-01) by Aska Electronic Co., Ltd., at an environmental
temperature of 25.degree. C., and then to
constant-current/constant-voltage charge with application of a
constant voltage of 2.9 V for 15 hours.
The negative electrode 4 was then sampled under an argon
atmosphere. The nonaqueous lithium power storage element was
disassembled under an argon atmosphere, and the negative electrode
4 was removed. Next, the obtained negative electrode 4 was immersed
in diethyl carbonate for 2 minutes or longer to remove the
nonaqueous electrolytic solution and lithium salt, and was
air-dried. The obtained negative electrode 4 was then immersed in a
mixed solvent comprising methanol and isopropanol for 15 hours to
inactivate the lithium ion intercalated in the negative electrode
active material, and was air-dried. Next, the obtained negative
electrode 4 was vacuum dried for 12 hours using a vacuum dryer
under conditions with a temperature of 170.degree. C., to obtain a
measuring sample. The obtained measuring sample was then used to
measure the BET specific surface area per unit volume of the
negative electrode active material layer of the negative electrode
after use and the mean pore size of the negative electrode active
material layer, by the methods described above, using a pore
distribution measuring apparatus by Yuasa Ionics Co., Ltd.
(AUTOSORB-1 AS-1-MP), with nitrogen as the adsorbate. The results
are shown in Table 5.
[Solid .sup.7Li-NMR Measurement of Positive Electrode]
The positive electrode of the obtained nonaqueous lithium power
storage element was used for solid .sup.7Li-NMR measurement of the
positive electrode active material layer.
First, the nonaqueous lithium power storage element produced as
described above was subjected to constant-current charge to 2.9 V
with a current of 50 mA, using a charge/discharge apparatus
(ACD-01) by Aska Electronic Co., Ltd., at an environmental
temperature of 25.degree. C., and then to
constant-current/constant-voltage charge with application of a
constant voltage of 2.9 V for 15 hours.
The positive electrode active material layer was then sampled under
an argon atmosphere. The nonaqueous lithium power storage element
was disassembled under an argon atmosphere, and the positive
electrode was removed. Next, the obtained positive electrode was
immersed in diethyl carbonate for 2 minutes or longer to remove the
nonaqueous electrolytic solution and lithium salt. After immersion
once more in diethyl carbonate under the same conditions, it was
air-dried.
The positive electrode active material layer was then sampled from
the positive electrode.
The obtained positive electrode active material layer was used as a
sample for solid .sup.7Li-NMR measurement. Measurement was
performed by the single pulse method, using an ECA700 (.sup.7Li-NMR
resonance frequency: 272.1 MHz) by JEOL RESONANCE Inc. as the
measuring apparatus, in a room temperature environment, with a
magic-angle spinning rotational speed of 14.5 kHz and the
irradiation pulse width set to a 45.degree. pulse. The observation
range was -400 ppm to 400 ppm, and the number of points was 4,096.
Measurement was performed with repeated latency of 10 seconds and
3,000 seconds, using the same measuring conditions other than the
repeated latency, such as the same number of scans and receiver
gain, and an NMR spectrum was obtained. A 1 mol/L aqueous lithium
chloride solution was used as the shift reference, and the shift
position measured separately as an external standard was defined as
0 ppm. During measurement of the 1 mol/L aqueous lithium chloride
solution, the single pulse method was used with an irradiation
pulse width set to a 45.degree. pulse, without rotation of the
sample.
The value of b/a was calculated by the method described above, from
the solid .sup.7Li-NMR spectrum of the positive electrode active
material layer obtained by the method described above. The results
are shown in Table 5.
Examples 37 to 59 and Comparative Examples 10 and 11
Positive electrode precursors were produced in the same manner as
Example 36, except that the positive electrode active materials,
the lithium compounds and their mean particle diameters, and the
parts by weight of the positive electrode active materials and
lithium compounds were as shown in Table 5. Nonaqueous lithium
power storage elements were produced and evaluated in the same
manner as Example 36, except that these positive electrode
precursors were used and combined with the negative electrodes
listed in Table 5, and the conditions for the lithium doping step
were as listed in Table 5. The results are shown in Table 5.
The results are summarized in Table 5 below.
TABLE-US-00005 TABLE 5 Negative electrode BET Solid Li.sup.7-NMR
specific Maxi- solace mum Lithium area of Mean peak amount negative
pore Positive electrode precursor position in electrode size of
Positive electrode Mean Positive Lithium in negative active
negative Mean Properties of particle electrode comp- spectral
electrode material electrode particle non-aqueous diameter active
ound Lithium doping step range of active layer active Solid
diameter lithium-type power Lithium of lithium material (parts
Temper- Volt- -10 ppm material per unit material Li.sup.7- of
lithium storage element Positive electrode com- compound (parts by
by ature age Time -35 ppm layer volume layer NMR compound E/V.sub.1
Ra F Rb/ active material pound (.mu.m) weight) weight) (.degree.
C.) (V) (hr) Name (ppm) (mmol/g) (m.sup.2/cc) (nm) b/a (.mu.m)
(Wh/L) (.O- MEGA.F) Ra Example 36 Activated carbon 1
Li.sub.2CO.sub.3 1.8 57.5 30.0 50 4.5 12 Neg. electrode 4 16 0.92
1.0 2.3 1.52 1.6 33 1.96 1.64 Example 37 Activated carbon 2
Li.sub.2CO.sub.3 1.8 57.5 30.0 50 4.5 12 Neg. electrode 4 18 1.24
1.2 2.5 1.72 1.6 36 1.82 1.39 Example 38 Activated carbon 2
Li.sub.2CO.sub.3 1.8 57.5 30.0 50 4.5 12 Neg. electrode 5 18 1.63
3.7 3.7 1.70 1.6 36 1.54 1.19 Example 39 Activated carbon 2
Li.sub.2CO.sub.3 1.8 57.5 30.0 50 4.5 12 Neg. electrode 6 11 2.30
8.5 8.3 1.68 1.7 35 1.18 1.07 Example 40 Activated carbon 2
Li.sub.2CO.sub.3 1.8 57.5 30.0 50 4.5 12 Neg. electrode 7 11 2.79
13.6 14.5 1.66 1.7 34 1.21 1.09 Example 41 Activated carbon 2
Li.sub.2CO.sub.3 1.8 57.5 30.0 50 4.5 12 Neg. electrode 8 9 5.93
41.5 4.0 1.65 1.7 35 1.22 1.12 Example 42 Activated carbon 2
Li.sub.2CO.sub.3 1.8 57.5 30.0 50 4.5 12 Neg. electrode 9 22 1.51
2.1 2.3 1.61 1.7 36 1.62 1.15 Example 43 Activated carbon 2
Li.sub.2CO.sub.3 1.8 57.5 30.0 50 4.5 12 Neg. electrode 10 14 2.48
7.5 5.8 1.82 1.7 36 1.20 1.06 Example 44 Activated carbon 2
Li.sub.2CO.sub.3 1.8 57.5 30.0 50 4.5 12 Neg. electrode 11 8 7.33
42.8 3.4 1.79 1.7 36 1.36 1.39 Example 45 Activated carbon 2
Li.sub.2CO.sub.3 1.8 57.5 30.0 50 4.5 12 Neg. electrode 12 10 6.72
19.3 15.5 1.74 1.5 34 1.29 1.20 Example 46 Activated carbon 2
Li.sub.2CO.sub.3 1.8 57.5 30.0 50 4.5 12 Neg. electrode 13 7 7.30
47.3 19.7 1.68 1.5 33 1.42 1.43 Example 47 Activated carbon 2
Li.sub.2CO.sub.3 1.1 72.5 15.0 50 4.5 12 Neg. electrode 6 8 0.21
9.3 8.4 1.06 0.9 35 2.89 1.95 Example 48 Activated carbon 2
Li.sub.2CO.sub.3 1.8 67.5 20.0 25 4.4 12 Neg. electrode 6 9 0.73
9.2 8.4 1.17 1.7 35 2.23 1.56 Example 49 Activated carbon 2
Li.sub.2CO.sub.3 1.8 42.5 45.0 50 4.5 35 Neg. electrode 6 21 7.67
8.1 8.1 3.55 1.6 35 1.47 1.25 Example 50 Activated carbon 2
Li.sub.2CO.sub.3 1.8 67.5 20.0 25 4.4 10 Neg. electrode 10 11 0.51
8.2 6.2 1.24 1.7 36 2.44 1.50 Example 51 Activated carbon 2
Li.sub.2CO.sub.3 1.8 42.5 45.0 50 4.5 24 Neg. electrode 10 21 7.22
7.2 5.6 3.65 1.6 36 1.42 1.27 Example 52 Activated carbon 2
Li.sub.2CO.sub.3 1.8 27.5 60.0 50 4.6 48 Neg. electrode 10 24 8.30
7.1 5.5 5.03 1.6 36 1.68 1.30 Example 53 Activated carbon 2
Li.sub.2CO.sub.3 1.1 72.5 15.0 50 4.5 12 Neg. electrode 12 5 0.88
21.2 15.9 1.08 0.6 34 2.01 1.87 Example 54 Activated carbon 2
Li.sub.2CO.sub.3 1.8 42.5 45.0 50 4.5 35 Neg. electrode 12 13 7.83
18.9 15.2 3.46 1.6 34 1.53 1.33 Example 55 Activated carbon 2
Li.sub.2CO.sub.3 1.8 27.5 60.0 50 4.6 72 Neg. electrode 12 18 9.46
18.2 14.8 4.76 1.5 34 2.47 1.42 Example 56 Activated carbon 2
Li.sub.2CO.sub.3 6.0 57.5 30.0 50 4.5 12 Neg. electrode 6 12 3.64
8.5 8.3 2.98 5.8 35 1.53 1.07 Example 57 Activated carbon 2
Li.sub.2CO.sub.3 9.4 57.5 30.0 50 4.5 12 Neg. electrode 6 13 4.21
8.5 8.3 3.78 9.3 35 1.79 1.05 Example 58 Activated carbon 2
Li.sub.2O 2.0 57.5 30.0 50 4.5 12 Neg. electrode 6 10 2.11 8.5 8.3
1.69 1.9 35 1.28 1.15 Example 59 Activated carbon 2 LiOH 2.1 57.5
30.0 50 4.5 12 Neg. electrode 6 10 2.00 8.5 8.3 1.75 2.0 35 1.34
1.13 Comp. Activated carbon 2 Li.sub.2CO.sub.3 0.6 77.5 10.0 25 4.4
8 Neg. electrode 6 3 0.06 9.3 8.4 1.01 0.4 35 3.68 2.53 Example 10
Comp. Activated carbon 2 Li.sub.2CO.sub.3 0.6 77.5 10.0 25 4.4 8
Neg. electrode 10 6 0.08 8.2 6.2 1.02 0.4 36 3.15 2.25 Example
11
From Examples 36 to 59 and Comparative Examples 10 and 11 it is
seen that by adding a graphite-based carbon material as a negative
electrode active material to the negative electrode, having a
maximum peak between 4 ppm to 30 ppm in the spectral range from -10
ppm to 35 ppm in the solid .sup.7Li-NMR spectrum of the negative
electrode active material layer, and adjusting the lithium amount
to a specified range, as calculated by the peak area from 4 ppm to
30 ppm, it is possible for a nonaqueous lithium power storage
element using the negative electrode to exhibit low resistance (i.
e., a high input/output characteristic) and a high high-load
charge/discharge cycle characteristic.
Example 60
[Preparation of Positive Electrode Precursor]
After mixing 57.5 parts by weight of activated carbon 1 obtained in
Example 1, 30.0 parts by weight of lithium carbonate having a mean
particle diameter of 2.3 .mu.m, as a lithium compound, 3.0 parts by
weight of Ketchen black, 1.5 parts by weight of PVP
(polyvinylpyrrolidone), 8.0 parts by weight of PVdF (polyvinylidene
fluoride) and NMP (N-methylpyrrolidone), the mixture was dispersed
using a FILMIX.RTM. thin-film spinning high-speed mixer by Primix
Corp., under conditions with a circumferential speed of 17 m/s, to
obtain a coating solution. The viscosity (.eta.b) and TI value of
the obtained coating solution were measured using a TVE-35H E-type
viscometer by Toki Sangyo Co., Ltd. As a result, the viscosity
(.eta.b) was 2,321 mPas and the TI value was 2.0. The degree of
dispersion of the obtained coating solution was measured using a
fineness gauge by Yoshimitsu Seiki Co. As a result, the granularity
was 35 .mu.m. The coating solution was coated onto one or both
sides of an aluminum foil with a thickness of 15 .mu.m and without
through-holes, using a die coater by Toray Engineering Co., Ltd.
under conditions with a coating speed of 1 m/s, and dried at a
drying temperature of 100.degree. C. to obtain a positive electrode
precursor (hereunder referred to as "single-sided positive
electrode precursor" and "double-sided positive electrode
precursor", respectively). The obtained positive electrode
precursor was pressed using a roll press under conditions with a
pressure of 4 kN/cm and a pressed portion surface temperature of
25.degree. C.
[Preparation of Negative Electrode]
[Preparation Example for Negative Electrode 14]
Artificial graphite 5 having a mean particle diameter of 0.7 .mu.m
and a BET specific surface area of 15.2 m.sup.2/g, used in an
amount of 150 g, was placed into a stainless steel mesh basket and
set on a stainless steel vat containing 30 g of coal pitch 3
(softening point: 135.degree. C.), and both were set in an electric
furnace (furnace inner usable dimension: 300 mm.times.300
mm.times.300 mm). This was increased in temperature to
1,200.degree. C. for 8 hours under a nitrogen atmosphere, and kept
at the same temperature for 4 hours for thermal reaction to obtain
composite carbon material 12. The obtained composite carbon
material 12 was cooled to 60.degree. C. by natural cooling, and
then removed out from the electric furnace.
The mean particle diameter and BET specific surface area of the
obtained composite carbon material 12 were measured by the same
methods as described above. The results are shown in Table 6.
Composite carbon material 12 was then used as a negative electrode
active material to produce negative electrode 14.
After mixing 80 parts by weight of composite carbon material 12, 8
parts by weight of acetylene black, 12 parts by weight of PVdF
(polyvinylidene fluoride) and NMP (N-methylpyrrolidone), the
mixture was dispersed using a FILMIX.RTM. thin-film spinning
high-speed mixer by Primix Corp., under conditions with a
circumferential speed of 15 m/s, to obtain a coating solution. The
viscosity (.eta.b) and TI value of the obtained coating solution
were measured using a TVE-35H E-type viscometer by Toki Sangyo Co.,
Ltd. As a result, the viscosity (.eta.b) was 2,274 mPas and the TI
value was 4.2. The coating solution was coated onto both sides of
an electrolytic copper foil with a thickness of 10 .mu.m and
without through-holes, using a die coater by Toray Engineering Co.,
Ltd. under conditions with a coating speed of 1 m/s, and dried at a
drying temperature of 85.degree. C. to obtain negative electrode 14
(hereunder also referred to as "double-sided negative electrode").
The obtained negative electrode 14 was pressed using a roll press
under conditions with a pressure of 4 kN/cm, a pressed portion
surface temperature of 25.degree. C., and a gap of 30 .mu.m between
the press rolls. The film thickness of the obtained negative
electrode 14 was measured at 10 arbitrary locations of negative
electrode 14, using a Linear Gauge Sensor GS-551 by Ono Sokki Co.,
Ltd. The thickness of the copper foil was subtracted from the mean
value of the measured film thickness, to determine the film
thickness of the negative electrode active material layer of
negative electrode 14. As a result, the film thickness of the
negative electrode active material layer of negative electrode 14
was 20 .mu.m for each side.
[Preparation Example for Negative Electrodes 15 to 32]
Negative electrode active materials were produced and evaluated in
the same manner as the preparation example for negative electrode
14, except that the base materials and their parts by weight, the
coal-based pitches and their parts by weight, and the heat
treatment temperatures were adjusted as shown in Table 6. Also,
negative electrodes were produced and evaluated in the same manner
as the preparation example for negative electrode 14, except that
preparation was using the negative electrode active materials
obtained as described above, with the coating solutions listed in
Table 6, and pressing of the formed negative electrodes was under
the pressing conditions listed in Table 6. The results are shown in
Table 6.
TABLE-US-00006 TABLE 6 Negative Pressing conditions electrode
Negative electrode active material Surface active Weight Mean
temper- material Starting material Heat ratio of particle BET ature
Gap layer Base material Pitch 3 treatment carbona- diam- specific
Pres- of between film Amount Amount temper- ceous eter surface sure
pressing press thickness (parts by (parts by ature material r.sub.a
area (kN/ section rolls per side Name Type weight) weight)
(.degree. C.) (weight %) (.mu.m) (m.sup.2/g) cm) (.degree. C.)
(.mu.m) (.mu.m) Negative Composite carbon Artificial 150 30 1200 7
1.2 11.3 4 25 30 20 electrode 14 material 12 graphite 5 Negative
Composite carbon Artificial 150 30 1200 7 1.2 11.3 2 25 30 20
electrode 15 material 12 graphite 5 Negative Composite carbon
Artificial 150 15 1200 2 4.9 7.4 6 140 30 20 electrode 16 material
13 graphite 6 Negative Composite carbon Artificial 150 15 1200 2
4.9 7.4 5 140 30 20 electrode 17 material 13 graphite 6 Negative
Composite carbon Artificial 150 15 1200 2 4.9 7.4 5 25 30 20
electrode 18 material 13 graphite 6 Negative Composite carbon
Artificial 150 15 1200 2 4.9 7.4 4 25 30 20 electrode 19 material
13 graphite 6 Negative Composite carbon Artificial 150 15 1200 2
9.8 1.2 5 140 30 20 electrode 20 material 14 graphite 7 Negative
Composite carbon Artificial 150 15 1200 2 9.8 1.2 4 140 30 20
electrode 21 material 14 graphite 7 Negative Composite carbon
Artificial 150 15 1200 2 9.8 1.2 4 25 30 20 electrode 22 material
14 graphite 7 Negative Composite carbon Artificial 150 15 1200 2
9.8 1.2 2 25 30 20 electrode 23 material 14 graphite 7 Negative
Composite carbon Artificial 150 15 1200 2 0.9 14.5 4 25 30 20
electrode 24 material 15 graphite 5 Negative Composite carbon
Natural 150 15 1200 2 5.8 8.2 6 140 30 20 electrode 25 material 16
graphite 4 Negative Composite carbon Natural 150 15 1200 2 5.8 8.2
5 140 30 20 electrode 26 material 16 graphite 4 Negative Composite
carbon Natural 150 15 1200 2 5.8 8.2 5 25 30 20 electrode 27
material 16 graphite 4 Negative Composite carbon Natural 150 15
1200 2 5.8 8.2 1 25 45 20 electrode 28 material 16 graphite 4
Negative Composite carbon Natural 150 15 1200 3 9.3 1.7 1 24 30 20
electrode 29 material 17 graphite 5 Negative Composite carbon High
area- 150 50 1200 17 2.7 48.2 4 140 30 20 electrode 30 material 18
to-weight ratio graphite 3 Negative Composite carbon High area- 150
50 1200 16 5.5 37.4 4 140 30 20 electrode 31 material 19 to-weight
ratio graphite 4 Negative Composite carbon High area- 150 50 1200
16 9.6 21.5 5 25 30 20 electrode 32 material 20 to-weight ratio
graphite 5
The starting materials in Table 6 are the following. Artificial
graphite 5: mean particle diameter=0.7 .mu.m, BET specific surface
area=15.2 m.sup.2/g Artificial graphite 6: mean particle
diameter=4.8 .mu.m, BET specific surface area=6.3 m.sup.2/g
Artificial graphite 7: mean particle diameter=9.8 .mu.m, BET
specific surface area=0.8 m.sup.2/g Natural graphite 4: mean
particle diameter=5.8 .mu.m, BET specific surface area=7.4
m.sup.2/g Natural graphite 5: mean particle diameter=9.2 .mu.m, BET
specific surface area=1.1 m.sup.2/g High specific surface area
graphite 3: mean particle diameter=2.4 .mu.m, BET specific surface
area=62.2 m.sup.2/g High specific surface area graphite 4: mean
particle diameter=5.4 .mu.m, BET specific surface area=45.7
m.sup.2/g High specific surface area graphite 5: mean particle
diameter=9.6 .mu.m, BET specific surface area=29.4 m.sup.2/g Pitch
3: coal-based pitch with softening point of 135.degree. C.
[Production of Nonaqueous Lithium Power Storage Element]
A plurality of nonaqueous lithium power storage elements were
produced in the same manner as Example 1, except for using the
positive electrode precursor obtained above and negative electrode
14, under the conditions described below under [Lithium doping
step] and [Aging step].
[Lithium Doping Step]
The obtained nonaqueous lithium power storage element was subjected
to initial charging by a method of constant-current charging using
a charge/discharge apparatus (TOSCAT-3100U) by Toyo System Co.,
Ltd., in a 55.degree. C. environment with a current value of 100 mA
until reaching a voltage of 4.5 V, followed by constant-voltage
charge at 4.5 V continued for 24 hours, for lithium doping of the
negative electrode 14.
[Aging Step]
The lithium-doped nonaqueous lithium power storage element was
subjected to a constant-current/constant-voltage charge step, with
constant-current discharge in a 25.degree. C. environment at 100 mA
until reaching a voltage of 2.0 V, followed by constant-current
charge at 300 mA until reaching a voltage of 4.4 V, and further
constant-current charge at 4.4 V for 20 hours.
[Evaluation of Nonaqueous Lithium Power Storage Elements]
One of the obtained nonaqueous lithium power storage elements was
subjected to [Electrostatic capacitance and RaF measurement] and
[High-load charge-discharge cycle test], as described above. The
results are shown in Table 7.
The remaining nonaqueous lithium power storage element was used for
[Measurement of mean distance between centers of gravity of voids
in cross-section of negative electrode active material layer of
negative electrode after use] described below, and for [Measurement
of mean particle diameter of lithium compound in positive
electrode], [Analysis of negative electrode active material layer
of negative electrode after use] and [Solid .sup.7Li-NMR
measurement of positive electrode], in the same manner as Example
36.
[Measurement of Mean Distance Between Centers of Gravity of Voids
in Cross-Section of Negative Electrode Active Material Layer of
Negative Electrode after Use]
Negative electrode 14 of the nonaqueous lithium power storage
element obtained as described above was used to measure the mean
distance between the centers of gravity of the voids of a
cross-section of the negative electrode active material layer of
the negative electrode after use.
First, the nonaqueous lithium power storage element produced as
described above was subjected to constant-current charge to 2.9 V
with a current of 50 mA, using a charge/discharge apparatus
(ACD-01) by Aska Electronic Co., Ltd., at an environmental
temperature of 25.degree. C., and then to
constant-current/constant-voltage charge with application of a
constant voltage of 2.9 V for 15 hours.
The negative electrode 14 was then sampled under an argon
atmosphere. The nonaqueous lithium power storage element was
disassembled under an argon atmosphere, and the negative electrode
14 was removed. Next, the obtained negative electrode 14 was
immersed in diethyl carbonate for 2 minutes or longer to remove the
nonaqueous electrolytic solution and lithium salt, and was
air-dried. The obtained negative electrode 14 was then immersed in
a mixed solvent comprising methanol and isopropanol for 15 hours to
inactivate the lithium ion intercalated in the negative electrode
active material, and was air-dried. Next, the obtained negative
electrode 14 was vacuum dried for 12 hours using a vacuum dryer
under conditions with a temperature of 170.degree. C., to obtain a
measuring sample.
The obtained measuring sample was subjected to BIB processing with
an argon ion beam using a cross-section polisher by JEOL Ltd. by
the method described above, under conditions with an acceleration
voltage of 4 kV, to form a cross-section of the negative electrode
active material layer of negative electrode 14.
Next, a scanning electron microscope (SU8220) by Hitachi
High-Technologies Corp. was used to obtain an SEM image of the
obtained cross-section of the negative electrode active material
layer, under the following conditions. Acceleration voltage: 1 kV
Emission current: 10 .mu.A Measurement magnification: 3,000x
Detector: Lower detector Working distance: 8.2 mm
An IP-1000 by Asahi Kasei Corp. (software: A-Zou Kun) was used for
image analysis of the SEM image of the obtained cross-section of
the negative electrode active material layer. A rectangular region
comprising only the cross-section of the negative electrode active
material layer of negative electrode 1 (20 .mu.m thickness
direction.times.50 .mu.m widthwise direction of the negative
electrode active material layer) was extracted from the obtained
SEM image (8 bit), and a median filter was used to remove the trace
noise in the image.
The extracted rectangular region was then binarized by the method
described above, and the mean distance between the centers of
gravity of the voids in the cross-section of the negative electrode
active material layer of negative electrode 14 was calculated. The
results are shown in Table 7.
Examples 61 to 86
Positive electrode precursors were produced in the same manner as
Example 60, except that the positive electrode active materials,
the lithium compounds and their mean particle diameters, and the
parts by weight of the positive electrode active materials and
lithium compounds were as shown in Table 7. Nonaqueous lithium
power storage elements were produced and evaluated in the same
manner as Example 60, except that these positive electrode
precursors were used and combined with the negative electrodes
listed in Table 7. The results are shown in Table 7 and Table 8
below.
Comparative Example 12
[Production of Positive Electrode Precursor]
After mixing 87.5 parts by weight of activated carbon 2 obtained in
Example 1, 3.0 parts by weight of Ketchen black, 1.5 parts by
weight of PVP (polyvinylpyrrolidone), 8.0 parts by weight of PVDF
(polyvinylidene fluoride) and NMP (N-methylpyrrolidone), the
mixture was dispersed using a FILMIX.RTM. thin-film spinning
high-speed mixer by Primix Corp., under conditions with a
circumferential speed of 17 m/s, to obtain a coating solution. A
positive electrode precursor was obtained in the same manner as
Example 60, except for using the coating solution obtained
above.
[Preparation and Evaluation of Nonaqueous Lithium Power Storage
Element]
Assembly, filling, impregnation and sealing of a nonaqueous lithium
power storage element were carried out in the same manner as
Example 60, except for using the obtained positive electrode
precursor, and the negative electrode comprising a metal lithium
foil corresponding to 280 mAh/g per unit weight of the negative
electrode active material, attached to the front side of the first
negative electrode active material layer of the negative electrode
listed in Table 7.
Next, for lithium doping, the obtained nonaqueous lithium power
storage element was stored for 30 hours in a thermostatic bath with
an environmental temperature of 45.degree. C., for ionization of
the metal lithium and doping in the negative electrode listed in
Table 7. The obtained nonaqueous lithium power storage element was
then subjected to aging and degassing in the same manner as Example
60, to produce a nonaqueous lithium power storage element, which
was evaluated. The results are shown in Table 7.
The results are summarized in Table 7 below.
TABLE-US-00007 TABLE 7 Negative electrode Mean distance BET between
specific centers of solace gravity of area of voids in negative
Mean Positive lectrode precursor cross-section electrode pore size
Positive electrode Mean Positive of negative active of negative
Mean particle electrode electrode material electrode particle
Properties of diameter active Lithium active layer active diameter
nonaqueous lithium-type of lithium material compound material per
unit material Solid of lithium power storage element Positive
electrode Lithium compound (parts by (parts by layer r.sub.p volume
layer .sup.7Li-NMR compound E/V.sub.1 Ra F active material compound
(.mu.m) weight) weight) Negative electrode (.mu.m) r.sub.p/r.sub.a
(m.sup.2/cc) (nm) b/a (.mu.m) (Wh/L) (.- OMEGA.F) Rb/Ra Example 60
Activated carbon 1 Li.sub.2CO.sub.3 2.3 57.5 30.0 Negative
electrode 14 1.1 0.92 12.5 10.5 1.53 2.1 32 2.59 1.87 Example 61
Activated carbon 2 Li.sub.2CO.sub.3 2.3 57.5 30.0 Negative
electrode 14 1.0 0.83 12.3 10.5 1.61 2.2 33 2.69 1.92 Example 62
Activated carbon 2 Li.sub.2CO.sub.3 2.3 57.5 30.0 Negative
electrode 15 1.3 1.08 12.8 10.4 1.59 2.2 32 2.01 1.60 Example 63
Activated carbon 2 Li.sub.2CO.sub.3 2.3 57.5 30.0 Negative
electrode 16 1.7 0.35 7.1 8.1 1.72 2.2 36 1.55 1.38 Example 64
Activated carbon 2 Li.sub.2CO.sub.3 2.3 57.5 30.0 Negative
electrode 17 2.1 0.43 7.4 8.2 1.70 2.1 35 1.19 1.10 Example 65
Activated carbon 2 Li.sub.2CO.sub.3 2.3 57.5 30.0 Negative
electrode 18 3,5 0.71 8.1 8.2 1.68 2.2 34 1.21 1.13 Example 66
Activated carbon 2 Li.sub.2CO.sub.3 2.3 57.5 30.0 Negative
electrode 19 4.8 0.98 9.3 8.3 1.66 2.1 34 1.45 1.21 Example 67
Activated carbon 2 Li.sub.2CO.sub.3 2.3 57.5 30.0 Negative
electrode 20 5.2 0.53 1.0 3.0 1.79 2.1 38 1.58 1.28 Example 68
Activated carbon 2 Li.sub.2CO.sub.3 2.3 57.5 30.0 Negative
electrode 21 6.3 0.64 1.2 3.2 1.77 2.1 37 1.87 1.43 Example 69
Activated carbon 2 Li.sub.2CO.sub.3 2.3 57.5 30.0 Negative
electrode 22 8.4 0.86 1.6 3.3 1.76 2.2 37 2.45 1.72 Example 70
Activated carbon 2 Li.sub.2CO.sub.3 2.3 57.5 30.0 Negative
electrode 23 9.8 1.00 2.1 3.5 1.68 2.1 36 2.88 1.93 Example 71
Activated carbon 2 Li.sub.2CO.sub.3 2.3 57,5 30.0 Negative
electrode 25 1.2 0.21 4.3 7,3 1.82 2.2 37 2.45 1.76 Example 72
Activated carbon 2 Li.sub.2CO.sub.3 2.3 57.5 30.0 Negative
electrode 26 2.3 0.40 6.8 8.3 1.77 2.2 35 1.15 1.09 Example 73
Activated carbon 2 Li.sub.2CO.sub.3 2.3 57.5 30.0 Negative
electrode 27 4.6 0.79 8.4 8.5 1.75 2.1 34 1.40 1.24 Example 74
Activated carbon 2 Li.sub.2CO.sub.3 2.3 57.5 30.0 Negative
electrode 28 7.1 1.22 9.2 9.0 1.72 2.2 32 2.05 1.58 Example 75
Activated carbon 2 Li.sub.2CO.sub.3 2.3 57.5 30.0 Negative
electrode 29 9.5 1.02 1.2 2.1 1.93 2.1 37 2.78 1.87 Example 76
Activated carbon 2 Li.sub.2CO.sub.3 2.3 57.5 30.0 Negative
electrode 30 1.6 0.59 43.5 18.4 2.06 1.7 32 2.21 1.53 Example 77
Activated carbon 2 Li.sub.2CO.sub.3 2.3 57.5 30.0 Negative
electrode 31 2.2 0.40 33.2 16.3 2.10 1.8 34 1.55 1.22 Example 78
Activated carbon 2 Li.sub.2CO.sub.3 2.3 57.5 30.0 Negative
electrode 32 8.5 0.89 22.3 15.4 2.14 1.8 31 2.89 1.89 Example 79
Activated carbon 2 Li.sub.2CO.sub.3 2.3 67.5 20.0 Negative
electrode 17 2.2 0.45 8.3 8.3 1.07 2.1 35 1.08 1.85 Example 80
Activated carbon 2 Li.sub.2CO.sub.3 2.3 42.5 45.0 Negative
electrode 17 2.1 0.43 7.0 7.8 3.59 2.1 35 1.43 1.14 Example 81
Activated carbon 2 Li.sub.2CO.sub.3 2.3 27.5 60.0 Negative
electrode 17 2.0 0.41 6.4 7.5 5.50 2.1 35 2.05 1.31 Example 82
Activated carbon 2 Li.sub.2CO.sub.3 0.4 57.5 30.0 Negative
electrode 17 2.2 0.45 8.4 8.5 1.12 0.2 35 1.12 1.79 Example 83
Activated carbon 2 Li.sub.2CO.sub.3 6.4 57.5 30.0 Negative
electrode 17 2.0 0.41 7.2 7.7 3.35 6.2 35 1.35 1.12 Example 84
Activated carbon 2 Li.sub.2CO.sub.3 9.0 57.5 30.0 Negative
electrode 17 2.0 0.41 6.8 7.5 3.69 8.7 35 1.52 1.16 Example 85
Activated carbon 2 Li.sub.2O 2.3 57.5 30.0 Negative electrode 17
2.1 0.43 7.4 8.1 1.82 2.1 35 1.22 1.15 Example 86 Activated carbon
2 LiOH 2.3 57.5 30.0 Negative electrode 17 2.1 0.43 7.4 8.0 1.93
2.2 35 1.20 1.14 Comp. Activated carbon 2 None -- 87.5 0.0 Negative
electrode 24 0.8 0.89 17.9 11.7 1.00 -- 33 3.04 2.45 Example 12
From Examples 60 to 86 and Comparative Example 12 it is seen that
by adding a lithium compound other than the positive electrode
active material to the positive electrode and adjusting the mean
distance between the centers of gravity of the voids obtained by
SEM of a cross-section of the negative electrode active material
layer to within a specific range in the nonaqueous lithium power
storage element, it is possible to exhibit low resistance (that is,
a high input/output characteristic) and a high high-load
charge/discharge cycle characteristic.
Reference Example 1
Negative electrode 17, before being incorporated in a nonaqueous
lithium power storage element, was used for [Measurement of mean
distance between centers of gravity of voids of cross-section of
the negative electrode active material layer of negative electrode
before use].
[Measurement of Mean Distance Between Centers of Gravity of Voids
of Cross-Section of the Negative Electrode Active Material Layer of
Negative Electrode Before Use]
The negative electrode 17 before incorporation into a nonaqueous
lithium power storage element was used as a measuring sample for
formation of a cross-section of the negative electrode active
material layer and SEM observation, in the same manner as Example
60. The obtained SEM image was used for image analysis in the same
manner as Example 60, and the mean distance between the centers of
gravity of the voids of the cross-section of the negative electrode
active material layer of negative electrode 17 was calculated. The
results are shown in Table 8.
Reference Example 2
A nonaqueous lithium power storage element was produced in the same
manner as Example 64, and was used for [Measurement of mean
distance between centers of gravity of voids in cross-section of
negative electrode active material layer of negative electrode
after use], by the method described below.
[Measurement of Mean Distance Between Centers of Gravity of Voids
in Cross-Section of Negative Electrode Active Material Layer of
Negative Electrode after Use]
Negative electrode 17 of the nonaqueous lithium power storage
element obtained as described above was used to measure the mean
distance between the centers of gravity of the voids of a
cross-section of the negative electrode active material layer of
the negative electrode after use.
First, the nonaqueous lithium power storage element produced as
described above was subjected to constant-current charge to 2.9 V
with a current of 50 mA, using a charge/discharge apparatus
(ACD-01) by Aska Electronic Co., Ltd., at an environmental
temperature of 25.degree. C., and then to
constant-current/constant-voltage charge with application of a
constant voltage of 2.9 V for 15 hours.
The negative electrode 17 was then sampled under an argon
atmosphere. The nonaqueous lithium power storage element was
disassembled under an argon atmosphere, and the negative electrode
17 was removed. Next, the obtained negative electrode 17 was
immersed in diethyl carbonate for 2 minutes or longer to remove the
nonaqueous electrolytic solution and lithium salt, and was
air-dried. The obtained negative electrode 17 was then used as the
working electrode and metal lithium as the counter electrode and
reference electrode, and these were immersed in the nonaqueous
electrolytic solution prepared in Example 60 under an argon
atmosphere, to fabricate an electrochemical cell. Using a
charge/discharge apparatus (TOSCAT-3000U) by Toyo System Co., Ltd.,
the obtained electrochemical cell was subjected to constant-current
charge at a current of 10 mA until reaching a voltage of 2.5 V
(i.e., until the negative electrode potential of the negative
electrode 17 (vs. Li/Li.sup.+) reached 2.5 V), followed by
constant-current/constant-voltage charge with application of a
constant voltage of 2.5 V for 15 hours. The charging referred to
here is the procedure of releasing lithium ions from the negative
electrode 17. Next, the negative electrode 17 was removed from the
electrochemical cell under an argon atmosphere, and immersed in
diethyl carbonate for 2 minutes or longer to remove the nonaqueous
electrolytic solution and lithium salt, and then air-dried. Next,
the obtained negative electrode 17 was vacuum dried for 12 hours
using a vacuum dryer under conditions with a temperature of
170.degree. C., to obtain a measuring sample.
The obtained measuring sample was used for formation of a
cross-section of the negative electrode active material layer and
SEM observation in the same manner as Example 60. The obtained SEM
image was used for image analysis in the same manner as Example 60,
and the mean distance between the centers of gravity of the voids
of the cross-section of the negative electrode active material
layer of negative electrode 17 was calculated. The results are
shown in Table 8.
TABLE-US-00008 TABLE 8 Negative electrode Mean distance between
centers of gravity of voids in cross-section of negative electrode
active material Name layer r.sub.p (.mu.m) r.sub.p/r.sub.a Example
64 Negative electrode 17 2.1 0.43 Reference Negative electrode 17
2.2 0.45 Example 1 Reference Negative electrode 17 2.1 0.43 Example
2
From Example 64 and Reference Examples 1 and 2 it is seen that
similar results are obtained whether before or after being
incorporated into a nonaqueous lithium power storage element, and
regardless of differences in the pretreatment method for the
measuring sample in [Measurement of mean distance between centers
of gravity of voids in cross-section of negative electrode active
material layer of negative electrode after use].
INDUSTRIAL APPLICABILITY
A nonaqueous lithium power storage element using a negative
electrode of the invention may be suitably used, for example, in
the field of hybrid drive systems for automobiles, in which
automobile internal combustion engines, fuel cells or motors are
used in combination, and in assist applications for instantaneous
electric power peaks, for example.
* * * * *